Shingon Buddhism (真言宗 Shingon-shū ) is one of the mainstream major schools of Japanese Buddhism and one of the few surviving Esoteric Buddhist lineages that started in the 3rd to 4th century AD that originally spread from India to China through traveling monks such as Vajrabodhi and Amoghavajra. The esoteric teachings would later flourish in Japan under the auspices of a Buddhist monk named Kūkai (空海), who traveled to Tang Dynasty China to acquire and request transmission of the esoteric teachings. For that reason, it is often called Japanese Esoteric Buddhism, or Orthodox Esoteric Buddhism. The word "Shingon" is the Japanese reading of the Kanji for the Chinese word Zhēnyán (真言), literally meaning "True Words", which in turn is the Chinese translation of the Sanskrit word mantra (मन्त्र).
Shingon Buddhist doctrine and teachings arose during the Heian period (794-1185) when a Buddhist monk named Kūkai traveled to China in 804 to study Esoteric Buddhist practices in the city of Xi'an (西安) (then called Chang-an), at Qinglong Temple (青龍寺, Blue Dragon Temple) under Master Huiguo, a favorite student of the legendary Amoghavajra and returned to Japan as his lineage and Dharma successor. Shingon followers usually address Kūkai as Kōbō-Daishi (弘法大師; lit. "Great Master of the Propagation of Dharma") or O-Daishi-sama (お大師様; "The Great Master"), the posthumous name given to him years after his death by Emperor Daigo.
Before he went to China, Kūkai had been an independent Buddhist monk in Japan for over a decade. He was extremely well versed in classical Chinese prose, calligraphy and Buddhist sutras. Esoteric Buddhism was not considered to be a different sect or school yet at that time. Huiguo was the first person to gather the still scattered elements of Indian and Chinese Esoteric Buddhism into a cohesive system. A Japanese monk named Gonsō (勤操) had brought back to Japan from China an esoteric mantra of Akasagarbha known as the Kokūzō-gumonjihō (虚空蔵求聞持法, lit. Akasagarbha Memory Retention Practice) that was translated from Sanskrit into Chinese by Śubhakarasiṃha (Zenmui-Sanzō 善無畏三蔵). When Kūkai was 22, he learned this from Gonsō and would go into the forests of Shikoku (四国) regularly to practice this mantra for long periods of time. He persevered in this mantra practice for seven years and mastered it. According to tradition, this practice brought him siddhis of superhuman memory retention and learning ability. Kūkai would later praise the power and efficacy of this Kokuzō-gumonjiho practice, crediting it with enabling him to remember all of Huiguo's teachings in only three months.
His respect for the Bodhisattva Akasagarbha was so great that he would regard Akasagarbha as his Honzon (本尊) or main deity, the single most important divinity to him, for the rest of his life. It was also during this period of intense mantra practice that he dreamt of a man telling him to seek out the Mahavairocana Tantra for the doctrine that he sought. The Mahavairocana Tantra had only recently been made available in Japan. He was able to obtain a copy in Chinese but large portions were in Siddham Sanskrit, which he did not know, and even the Chinese portions were too arcane for him to understand. He believed that this teaching was a door to the truth he sought, but he was unable to fully comprehend it and no one in Japan could help him. Thus, he made his resolve to travel to China to spend the time necessary to fully understand the Mahavairocana Tantra.
When Kūkai reached China and first met Huiguo on the fifth month of 805, Huiguo was age sixty and on the verge of death from a long spate of illness. Huiguo exclaimed to Kūkai in Chinese (in paraphrase), "At last, you have come! I have been waiting for you! Quickly, prepare yourself for initiation into the mandalas!" Huiguo had foreseen that Esoteric Buddhism would not survive in India and China in the near future and that it was Kukai's destiny to see it continue in Japan. In the short space of three months, Huiguo initiated and taught Kūkai everything he knew on the doctrines and practices of the Mandala of the Two Realms as well as mastery of Siddham Sanskrit and (presumably to be able to communicate with Master Huiguo) Chinese. Huiguo declared Kūkai to be his final disciple and proclaimed him a Dharma successor, giving the lineage name Henjō-Kongō (遍照金剛, biànzhào jīngāng), meaning "illuminating adamantine vajra".
In the twelfth month of the same year, Huiguo died and was appropriately buried next to his master Amoghavajra.
More than one thousand of his disciples gathered for his funeral. The honor of writing his funerary inscription on their behalf was given to Kūkai. Kukai returned to Japan after Huiguo's death. If he had not, Esoteric Buddhism might not have survived because 35 years after Huiguo's death in the year 840, the infamous Emperor Wuzong of Tang assumed the throne. An avid Taoist, the new emperor despised Buddhism and thought of monks as useless tax-evaders. In 845 he ordered the destruction of 4,600 Buddhist monasteries and 40,000 temples. Around 250,000 Buddhist monks and nuns had to give up their monastic lives. Wuzong cited that Buddhism was an alien religion and promoted indigenous Taoism zealously. Shortly, he was assassinated by his own inner circle but the damage had been done. Ancient Chinese Buddhism never fully recovered from the persecution, and many esoteric elements were infused into other Buddhist sects and traditions.
After returning to Japan, Kūkai collated and systematized all that he had learned from Huiguo into a cohesive doctrine of pure esoteric Buddhism that would become the basis for the Shingon school. Originally, he did not establish his doctrine as a separate school and did not specifically name it "Shingon-Shū", as it is now known. It would be the Emperor Junna, who favored Kūkai and Esoteric Buddhism who would coin the term "Shingon-Shū" (真言宗; "The True Word School") in his imperial decree which officially declared Tō-ji (東寺) Temple in Kyoto as a purely Shingon temple that would perform official rites for the state. Kūkai actively took on disciples and offered transmission until his death in 835 at the age of 61.
Kūkai's first established monastery was in Kōya-san (高野山; "Mount Kōya"), which has since become the base and a place of spiritual retreat for Shingon practitioners.
Shingon enjoyed immense popularity during the Heian Period (平安時代), particularly among the Heian nobility, and contributed greatly to the art and literature of the time, as well as influencing other communities, such as the Tendai School (天台宗) on Mount Hiei (Hiei-zan 比叡山).
Also, Shingon's emphasis on ritual found support in the Kyoto nobility, particularly the Fujiwara clan (藤原氏). This favor allotted Shingon several politically powerful temples in the capital, where rituals for the Imperial Family and nation were regularly performed. Many of these temples such as Tō-ji (東寺) and Daigo-ji (醍醐寺) in the South of Kyōto (京都) and Jingo-ji (神護寺) and Ninna-ji (仁和寺) in the Northwest became ritual centers establishing their own particular ritual lineages.
The Shingon lineage is an ancient transmission of esoteric Buddhist doctrine that began in India and then spread to China and Japan. Shingon is the name of this lineage in Japan, but there are also esoteric schools in China, Korea, Taiwan and Hong Kong that consider themselves part of this lineage (as the originators of the Esoteric teachings) and universally recognize Kūkai as their eighth patriarch. This is why sometimes the term "Orthodox Esoteric Buddhism" is used instead.
Shingon or Orthodox Esoteric Buddhism maintains that the expounder of the doctrine was originally the Universal Buddha Mahavairocana but the first human to receive the doctrine was Nagarjuna in India. The tradition recognizes two groups of eight great patriarchs - one group of lineage holders and one group of great expounders of the doctrine.
The Eight Great Lineage Patriarchs (Fuho-Hasso 付法八祖)
- Mahavairocana (Dainichi-Nyorai 大日如来)
- Vajrasattva (Kongō-Satta 金剛薩埵)
- Nagarjuna (Ryūju-Bosatsu 龍樹菩薩) - received the Mahavairocana Tantra from Vajrasattva inside an Iron Stupa in Southern India)
- Nagabodhi (Ryūchi-Bosatsu 龍智菩薩)
- Vajrabodhi (Kongōchi-Sanzō 金剛智三蔵)
- Amoghavajra (Fukūkongō-Sanzō 不空金剛三蔵)
- Huiguo (Keika-Ajari 恵果阿闍梨)
- Kōbō-Daishi (弘法大師)
The Eight Great Doctrine-Expounding Patriarchs (Denji-Hasso 伝持八祖)
- Nagarjuna (Ryūju-Bosatsu 龍樹菩薩)
- Nagabodhi (Ryūchi-Bosatsu 龍智菩薩)
- Vajrabodhi (Kongōchi-Sanzō 金剛智三蔵)
- Amoghavajra (Fukūkongō-Sanzō 不空金剛三蔵)
- Śubhakarasiṃha (Zenmui-Sanzō 善無畏三蔵)
- I-Hsing (Ichigyō-Zenji 一行禅師)
- Huiguo (Keika-Ajari 恵果阿闍梨)
- Kōbō-Daishi (弘法大師)
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Like the Tendai School that branched into the Pureland School (Jōdo shū 浄土宗) and the Nichiren School (Nichiren-kei sho shūha 日蓮系諸宗派) during the Kamakura period, Shingon divided into two major schools – the old school, Kogi Shingon (古義真言宗, lit. Ancient Shingon school), and the new school, Shingi Shingon (新義真言宗, lit. Reformed Shingon school).
This division primarily arose out of a political dispute between Kakuban (覚鑁), known posthumously as Kōgyō-Daishi (興教大師), and his faction of priests centered at the Denbō-in (伝法院) and the leadership at Kongōbuji (金剛峰寺), the head of Mount Kōya and the authority in teaching esoteric practices in general. Kakuban, who was originally ordained at Ninnaji (仁和寺) in Kyōto, studied at several temple-centers including the Tendai temple complex at Onjōji (園城寺) before going to Mount Kōya. Through his connections, he managed to gain the favor of high ranking nobles in Kyoto, which helped him to be appointed abbot of Mount Kōya. The leadership at Kongōbuji however, opposed the appointment on the premise that Kakuban had not originally been ordained on Mount Kōya.
After several conflicts, Kakuban and his faction of priests left the mountain for Mount Negoro (根来山) to the northwest, where they constructed a new temple complex, now known as Negoroji (根来寺). After the death of Kakuban in 1143, the Negoro faction returned to Mount Kōya. However in 1288, the conflict between Kongōbuji and the Denbō-in came to a head once again. Led by Raiyu, the Denbō-in priests once again left Mount Kōya, this time establishing their headquarters on Mount Negoro. This exodus marked the beginning of the Shingi Shingon School at Mount Negoro, which was the center of Shingi Shingon until it was sacked by the daimyo Toyotomi Hideyoshi (豊臣秀吉) in 1585.
Mahavairocana (Jap. Dainichi Nyorai 大日如來) as the central primordial buddha in Esoteric Buddhist doctrine is the true nature of all things and phenomena, the totality of reality in all form and formlessness, arising and non-arising. Though supernatural beings like Devas may be more powerful and live longer than humans, they are nevertheless afflicted by suffering and death.
When the Catholic missionary Francis Xavier first arrived in Japan, he was welcomed by the Shingon monks since he used the word Dainichi for the Christian God. As Xavier learned more about the religious nuances of the word, he changed it to Deusu from the Latin and Portuguese word Deus. At that point, the monks realized that Xavier was preaching a non-buddhist religion.
The teachings of Shingon are based on early Buddhist Tantras, the Mahavairocana Tantra (Jap. Dainichi-kyō 大日経), the Vajrasekhara Sutra (Kongōchō-kyō 金剛頂経), the Adhyardhaśatikā Prajñāpāramitā Sutra (Rishu-kyō 理趣経), and the Susiddhikara Sutra (Soshitsuji-kyō 蘇悉地経). These are the four principal texts of Esoteric Buddhism. They are all Tantras and not Sutras despite their names. The mystical Mahavairocana and Vajrasekhara teachings are expressed in the two main mandalas of Shingon, The Mandalas of The Two Realms - The Womb Realm (Skt. Garbhadhātu, Jap. Taizōkai 胎蔵界曼荼羅) mandala and the Diamond Realm (Skt. Vajradhātu, Jap. Kongōkai 金剛界曼荼羅) mandala. These two mandalas are considered to be a compact expression of the entirety of the Dharma, and form the root of Buddhism. In Shingon temples, these two mandalas are always mounted one on each side of the central altar. The Susiddhikara Sutra is largely a compendium of rituals. Buddhism is concerned with the rituals and meditative practices that lead to enlightenment. According to the Shingon doctrine, enlightenment is not a distant, foreign reality that can take aeons to approach but a real possibility within this very life, based on the spiritual potential of every living being, known generally as Buddha-nature. If cultivated, this luminous nature manifests as innate wisdom. With the help of a genuine teacher and through proper training of the body, speech, and mind, i.e. "The Three Mysteries" (Sanmitsu 三密), we can reclaim and liberate this enlightened capacity for the benefit of ourselves and others.
Kūkai also systematized and categorized the teachings he inherited from Huiguo into ten stages or levels of spiritual realisation. He wrote at length on the difference between exoteric mainstream Mahayana Buddhism and esoteric Tantric Buddhism. The differences between exoteric and esoteric can be summarised as:
- Esoteric teachings are preached by the Dharmakaya (Hosshin 法身) Buddha which Kūkai identifies as Mahavairocana (Dainichi Nyorai 大日如來). Exoteric teachings are preached by the Nirmanakaya (Ōjin 応身) Buddha, which in our world and aeon, is the historical Gautama Buddha (Shaka-muni 釈迦牟尼) or one of the Sambhoghakaya (Hōjin 報身) Buddhas.
- Exoteric Buddhism holds that the ultimate state of Buddhahood is ineffable, and that nothing can be said of it. Esoteric Buddhism holds that while nothing can be said of it verbally, it is readily communicated via esoteric rituals which involve the use of mantras, mudras, and mandalas.
- Kūkai held that exoteric doctrines were merely provisional, skillful means (Skt. Upāya) on the part of the Buddhas to help beings according to their capacity to understand the Truth. The esoteric doctrines by comparison are the Truth itself, and are a direct communication of the "inner experience of the Dharmakaya's enlightenment". A simple way to put it would be to say that when Gautama Buddha attained enlightenment in his earthly Nirmanakaya body, he realized that the Dharmakaya body is actually reality in its totality and that totality is Mahavairocana.
- Some exoteric schools in the late Nara and early Heian Japan held (or were portrayed by Shingon adherents as holding) that attaining Buddhahood is possible but requires a huge amount of time (three incalculable aeons) of practice to achieve, whereas esoteric Buddhism teaches that Buddhahood can be attained in this lifetime by anyone.
Kūkai held, along with the Chinese Huayan (Kegon 華嚴) and Tendai schools that all phenomena could be expressed as 'letters' in a 'World-Text'. Mantra, mudra, and mandala are special because they constitute the 'language' through which the Dharmakaya (i.e. Reality itself) communicates. Although portrayed through the use of anthropomorphic metaphors, Shingon does not see the Dharmakaya Buddha as a god, or creator (as a separate entity). The Dharmakaya is in fact a symbol for the true nature of reality and a representation of emptiness (Śūnyatā). Its important to note that,because of the interdependence between emptyness and form,Vairocana is also a representation of collective phenomena,of the universe itself.
Relationship to Vajrayāna 
When the teachings of Shingon Buddhism were brought to Japan, Esoteric Buddhism was still in its early stages in India. At this time, the terms Vajrayāna ("Diamond Vehicle") and Mantrayāna ("Mantra Vehicle") were not used for Esoteric Buddhist teachings. Instead, esoteric teachings were more typically referred to as Mantranaya, or the "Mantra System." According to Paul Williams, Mantranaya is the more appropriate term to describe the self-perception of early Esoteric Buddhism.
The primary difference between Shingon and Tibetan Buddhism is that there is no Anuttarayoga Tantra in Shingon. Shingon has what corresponds to the Kriyā, Caryā, and Yoga classes of tantras in Tibetan Buddhism. The Tibetan system of classifying tantras into four classes is not used in Shingon.
Anuttarayoga Tantras such as the Guhyasamaja Tantra, Yamantaka Tantra, Hevajra Tantra, Mahamaya Tantra, Cakrasamsarva Tantra, and the Kalachakra Tantra were developed at a later period of Esoteric Buddhism and are not used in Shingon.
Mahavairocana Tathagata 
In Shingon, Mahavairocana Tathagata (Dainichi Nyorai 大日如來) is the universal or Primordial Buddha that is the basis of all phenomena, present in each and all of them, and not existing independently or externally to them. The goal of Shingon is the realization that one's nature is identical with Mahavairocana, a goal that is achieved through initiation (for ordained followers), meditation and esoteric ritual practices. This realization depends on receiving the secret doctrines of Shingon, transmitted orally to initiates by the school's masters. The "Three Mysteries" of body, speech, and mind participate simultaneously in the subsequent process of revealing one's nature: the body through devotional gestures (mudra) and the use of ritual instruments, speech through sacred formulas (mantra), and mind through meditation.
Shingon places emphasis on the Thirteen Buddhas (十三仏), a grouping of various buddhas and bodhisattvas; however this is purely for lay Buddhist practice and Shingon priests generally make devotions to not just the Thirteen Buddhas.
- Acala Vidyaraja (Fudō Myōō 不動明王)
- Shakyamuni Buddha (Shaka-Nyorai 釈迦如来)
- Manjusri Bodhisattva (Monju-Bosatsu 文殊菩薩)
- Samantabhadra Bodhisattva (Fugen-Bosatsu 普賢菩薩)
- Kṣitigarbha Bodhisattva (Jizō-Bosatsu 地蔵菩薩)
- Maitreya Bodhisattva (Miroku-Bosatsu 弥勒菩薩)
- Bhaisajyaguru Buddha (Yakushi-Nyorai 薬師如來)
- Avalokitesvara Bodhisattva (Kannon-Bosatsu 観音菩薩)
- Mahasthamaprapta Bodhisattva (Seishi-Bosatsu 勢至菩薩 )
- Amitabha Buddha (Amida-Nyorai 阿弥陀如来)
- Akshobhya Buddha (Ashuku-Nyorai 阿閦如来)
- Mahavairocana Buddha (Dainichi-Nyorai 大日如来)
- Akasagarbha Bodhisattva (Kokūzō-Bosatsu 虚空蔵菩薩)
Mahavairocana is the Universal Principle which underlies all Buddhist teachings, according to Shingon Buddhism, so other Buddhist figures can be thought of as manifestations with certain roles and attributes. Each Buddhist figure is symbolized by its own Sanskrit "seed" letter as well.
Practices and features 
One feature that Shingon shares in common with Tendai, the only other school with esoteric teachings in Japan is the use of Siddham Sanskrit Seed-syllables or Bija along with anthropomorphic and symbolic representations, to express Buddhist deities in their Mandalas.
There are four types of mandalas:
• Mahā-Maṇḍala (大曼荼羅, Anthropomorphic Representation),
• Seed-Syllable Mandala or Dharma-Maṇḍala (法曼荼羅),
• Samaya-Maṇḍala (三昧耶曼荼羅, representations of the vows of the deities in the form of articles they hold or their mudras), and
• Karma-Maṇḍala (羯磨曼荼羅) representing the activities of the deities in the three-dimensional form of statues, etc.
An ancient Indian Sanskrit syllabary script known as Siddham (Jap. Shittan 悉曇 or Bonji 梵字) is used to write mantras. A core meditative practice of Shingon is Ajikan (阿字觀), "Meditating on the Letter 'A'", which uses the Siddham letter representing the sound "Ah." Other Shingon meditations are Gachirinkan (月輪觀, "Full Moon" visualization), Gojigonjingan (五字嚴身觀, "Visualization of the Five Elements arrayed in The Body" from the Mahavairocana Tantra) and Gosōjōjingan (五相成身觀, Pañcābhisaṃbodhi "Series of Five Meditations to attain Buddhahood" from the Vajrasekhara Sutra.
The essence of Shingon Mantrayana practice is to experience Reality by emulating the inner realization of the Dharmakaya through the meditative ritual use of mantra, mudra and visualization of mandala i.e. "The Three Mysteries" (Jap. Sanmitsu 三密). All Shingon followers gradually develop a teacher-student relationship, whereby a teacher learns the disposition of the student and teaches practices accordingly. For lay practitioners, there is no initiation ceremony beyond the Kechien Kanjō (結縁灌頂), which is normally offered only at Mount Koya but can also be offered by larger temples under masters permitted to transmit the empowerment. It is not required for all laypersons to take.
In the case of disciples wishing to train to become a Shingon acharya (Ajari 阿闍黎), it requires a period of academic and religious study, or formal training in a temple for a longer period of time, after having already taken the full precepts. Later the practitioner is able to undergo steps for training and an examination to be certified as a Shingon acharya. In either case, the stress is on finding a qualified and willing mentor who will guide the practitioner through the practice at a gradual pace. An acharya in Shingon is a committed and experienced teacher who is authorized to guide and teach practitioners. One must be an acharya for ten years at least before one can request to be tested at Mount Koya for the possibility to qualify as a Maha-acharya (Dai-Ajari 大阿闍黎), the highest rank of a Shingon practitioner and a qualified grand master.
Apart from prayers and reading of sutras, there are Mahayana Buddhist mantras and ritualistic meditative techniques that are available for laypersons to practice on their own. All practices require the devotee to undergo initiation or abhisheka in Sanskrit (Jap. Kanjō 灌頂) into each of these practices under the guidance of a qualified acharya before they may begin to learn and practice them. As with all schools of Buddhism, great emphasis is placed on the empowerment and oral transmission of teachings from teacher to student. Until the 1920s-40s (around the time of the arrival of Shingon outside Japan), nothing had ever been published on any Shingon or Mikkyo teachings in Japan or anywhere else. Everything was passed down orally for more than 1,100 years. Undergoing any Shingon practice on one's own without empowerment and guidance from a qualified master can be considered a serious offence of breaching samaya vows because it can potentially be harmful to the practitioner if not done in the proper manner.
Shingon/Esoteric Buddhism outside Japan 
East Asian Esoteric Buddhism is also practiced in the Japanese Tendai School (天台宗), founded in the same era as the Shingon School in the early 9th century (Heian period), although Tendai doctrine contain mostly exoteric teachings. The general term for Esoteric Buddhism in Japan is mikkyō (密教; literally "secret teachings"). In order to differentiate between the esoteric practices from the two schools, Shingon practices are also known as Tōmitsu (東密) while Tendai esoteric practices are known as Taimitsu (台密).
In China and countries with large Chinese populations such as Taiwan, Malaysia, and Singapore, Esoteric Buddhism is most commonly referred to as the Chinese term Mìzōng (密宗), or "Esoteric School." Traditions of Chinese Esoteric Buddhism are most commonly referred to as referred as Tángmì (唐密), "Tang Dynasty Esoterica," or Hànchuán Mìzōng (漢傳密宗), "Han Transmission Esoteric School" (Hànmì 漢密 for short), or Dōngmì (東密), "Eastern Esoterica," separating itself from Tibetan and Newar traditions. These schools more or less share the same doctrines as Shingon, and in some cases, Chinese monks have traveled to Japan to train and to be given esoteric transmission at Mount Koya and Mount Hiei.
See Zhenyan at encyclopedia.com on Chinese Esoteric Buddhism.
In the United States, Shingon is practiced at the branch temples of the Koyasan lineage, with locations in Los Angeles, Sacramento, Fresno, as well as many others in the state of Hawaii, Michigan and Washington. There are also branch temples of the Buzan lineage in Hong Kong and Vietnam.
Goma Fire Ritual 
The Homa (Jap. = Goma 護摩) Ritual of consecrated fire is unique to Esoteric Buddhism and is the most recognizable ritual defining Shingon amongst regular Japanese persons today. It is perhaps also the most mystical and cognitively powerful. It stems from the Vedic Agnihotra Ritual and is performed by qualified priests and acharyas for the benefit of individuals, the state or all sentient beings in general. The consecrated fire is believed to have a powerful cleansing effect spiritually and psychologically. The central deity invoked in this ritual is usually Acala (Fudō Myōō 不動明王). The ritual is performed for the purpose of destroying negative energies, detrimental thoughts and desires, and for the making of secular requests and blessings. In most Shingon temples, this ritual is performed daily in the morning or the afternoon. Larger scale ceremonies often include the constant beating of taiko drums and mass chanting of the mantra of Acala by priests and lay practitioners. Flames can sometimes reach a few meters high. The combination of the ritual's visuals and sounds can be trance-inducing and make for a profound experience.
The ancient Japanese religion of Shugendō (修験道) has also adopted the Goma Ritual except that theirs is usually performed at a much larger scale outdoors.
Today, there are very few books on Shingon in the West and until the 1940s, not a single book on Shingon had ever been published anywhere in the world, not even in Japan. Since this lineage was brought over to Japan from Tang Dynasty China over 1,100 years ago, its doctrines have always been closely guarded secrets, passed down orally through an initiatic chain and never written down. Throughout the centuries, except for the initiated, most of the Japanese common folk knew little about its secretive doctrines and the monks of this "Mantra School" except that besides performing the usual priestly duties of prayers, blessings and funeral rites for the public, they practiced only Mikkyō (密教）, literally "secret ways" in stark contrast to all other Buddhist schools, and were called upon to perform mystical rituals that were supposed to summon rain, improve harvests, exorcise demons, avert natural disasters, heal the sick and protect the state. The most powerful ones were thought to be able to render entire armies useless.
Even though the Tendai School also contains esoteric teachings in its doctrines, it is still essentially an exoteric Mahayana school at its core. Shingon teachings contain Esoteric teachings (such as the Rishukyo) as well as Exoteric teachings (such as the Diamond Sutra), and are in all likelihood also the most secretive Buddhist teachings in the world. As such, in-depth academic study will continue to prove difficult as it had been in the past.
The Shingon Pantheon 
A large number of deities of Vedic, Hindu and Indo-Aryan origins have been incorporated into Mahayana Buddhism and this synthesis is especially prominent in Esoteric Buddhism. Many of these deities have vital roles as they are regularly invoked by the practitioner for various rituals and homas/pujas. In fact, it is ironic that the worship of Vedic-era deities, especially Indra (Taishakuten 帝釈天), the "King of the Heavens," has declined so much in India but is yet so highly revered in Japan that there are probably more temples devoted to him there than there are in India. Chinese Taoist and Japanese Shinto deities were assimilated into Mahayana Buddhism as "Deva" class beings; for example to Chinese Mahayana Buddhists, Indra (synonymous with Śakra) is the Jade Emperor of Taoism. Agni (Katen 火天), another Vedic deity, is invoked at the start of every Shingon Goma Ritual. The average Japanese person may not know the names Saraswati or Indra but Benzaiten 弁財天 (Saraswati) and Taishakuten 帝釈天 (Indra) are household names that every Japanese person knows.
In Orthodox Esoteric Buddhism, divine beings are grouped into six classes.
- Buddhas (Butsu 仏)
- Bodhisattvas (Bosatsu 菩薩)
- Wisdom Kings or Vidyarajas (Myōō 明王)
- Deities or Devas (Ten 天)
- Avatars (Keshin 化身)
- Patriarchs (Soshi 祖師)
The Five Great Wisdom Kings
- Acala or Acalanatha (Fudō Myōō 不動明王) "The Immovable One" - Manifestation of Buddha Mahavairocana
- Amrtakundalin (Gundari Myōō 軍荼利明王) "The Dispenser of Heavenly Nectar" - Manifestation of Buddha Ratnasambhava
- Trailokyavijaya (Gōzanze Myōō 降三世明王) "The Conqueror of The Three Planes" - Manifestation of Buddha Akshobhya
- Yamāntaka (Daiitoku Myōō 大威徳明王) "The Defeater of Death" - Manifestation of Buddha Amitabha
- Vajrayaksa (Kongō Yasha Myōō 金剛夜叉明王) "The Devourer of Demons" - Manifestation of Buddha Amoghasiddi
Other well-known Wisdom Kings
- Ragaraja (Aizen Myōō 愛染明王)
- Mahamayuri (Kujaku Myōō 孔雀明王)
- Hayagriva (Batō Kannon 馬頭観音)
- Ucchusma (Ususama Myōō 烏枢沙摩明王)
- Atavaka (Daigensui Myōō 大元帥明王)
The Twelve Guardian Deities (Deva)
- Brahmā (Bonten 梵天) - Lord of the Heavens ; Guardian of the Heavens (upward direction)
- Indra (Taishakuten 帝釈天) - Lord of the Trāyastriṃśa Heaven and The Thirty Three Devas ; Guardian of the East
- Varuṇa (Suiten 水天) - Lord of Water ; Guardian of the West
- Vaishravana (Bishamonten 毘沙門天 or Tamonten 多聞天) - Lord of Wealth ; Guardian of the North
- Yama (Emmaten 焔魔天) - Lord of the Underworld ; Guardian of the South
- Agni (Katen 火天) - Lord of Fire ; Guardian of the South East
- Rakshasa (Rasetsuten 羅刹天) - Lord of Demons ; Guardian of the South West
- Shiva or Maheshvara (Daijizaiten 大自在天 or Ishanaten 伊舎那天) - Lord of The Desire Realms ; Guardian of the North East
- Vāyu (Fūten 風天)- Lord of Wind ; Guardian of the North West
- Sūrya (Nitten 日天) - Lord of the Sun
- Chandra (Gatten 月天) - Lord of the Moon
- Prthivi (Jiten 地天) - Lord of the Earth ; Guardian of the Earth (downward direction)
Other Important Deities (Deva)
- Marici (Marishi-Ten 摩里支天) - Patron deity of Warriors
- Mahakala (Daikokuten 大黒天) - Patron deity of Wealth
- Saraswati (Benzaiten 弁財天) - Patron deity of Knowledge, Art and Music
- Ganesha (Kangiten 歓喜天) Patron deity of Bliss and Remover of Obstacles
- Skanda (Idaten 韋駄天 or Kumaraten 鳩摩羅天) Protector of Buddhist Monasteries and Monks
Branches of Shingon 
- The Orthodox (Kogi) Shingon School (古義真言宗)
- Kōyasan (高野山真言宗)
- Chuin-Ryu Lineage (中院流)
- Tō-ji (東寺真言宗)
- Zentsūji-ha (真言宗善通寺派)
- Daigo-ha (真言宗醍醐派)
- Omuro-ha (真言宗御室派)
- Shingon-Ritsu (真言律宗)
- Daikakuji-ha (真言宗大覚寺派)
- Sennyūji-ha (真言宗泉涌寺派)
- Yamashina-ha (真言宗山階派)
- Shigisan (信貴山真言宗)
- Nakayamadera-ha (真言宗中山寺派)
- Sanbōshū (真言三宝宗)
- Sumadera-ha (真言宗須磨寺派)
- Tōji-ha (真言宗東寺派)
- Kōyasan (高野山真言宗)
- The Reformed (Shingi) Shingon School (新義真言宗)
See also 
- Caiger, Mason. A History of Japan, Revised Ed. pp. 106–107.
- Williams, Paul, and Tribe, Anthony. Buddhist Thought: A Complete Introduction to the Indian Tradition. 2000. p. 271
- "Jusan Butsu - The Thirteen Buddhas of the Shingon School <Internet>". Retrieved 5 July 2007.