Shingon enjoyed immense popularity during the Heian Period, 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 sect on Mt. Hiei.
Also, Shingon's emphasis on ritual found support in the Kyoto nobility, particularly the Fujiwara clan. 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 Toji, Ninnaji and Daigoji to the south of Kyoto became ritual centers establishing their own particular ritual lineages.
Like the Tendai School that branched into the Jōdo, Zen and Nichiren Schools in the Kamakura period, Shingon also divided into two major branches; Kogi Shingon, or "old Shingon," and Shingi Shingon, or "New Shingon." This division primarily arose out of a political dispute between Kakuban and his faction of priests centered at the Denbōe and the leadership at Kongōbuji, the head of Mt. Kōya. Kakuban, who was originally ordained at Ninnaji in Kyoto, studied at several temple-centers (including the Tendai temple complex at Onjiyōji) before going to Mt. Kōya. Through his connections, he managed to gain the favor of high ranking nobles in Kyoto, which helped him to be appointed abbot of Mt. Kōya. The leadership at Kongōbuji, however, opposed the appointment on the premise that Kakuban had not originally been ordained on Mt. Kōya. After several conflicts Kakuban and his faction of priests left the mountain for Mt. Negoro to the northwest, where they constructed a new temple complex, now known as Negoroji. After the death of Kakuban in 1143, the Negoro faction returned to Mt. Kōya. However in 1288, the conflict between Kongōbuji and the Denbōe came to a head once again. Led by Raiyu, the Denbōe priests once again left Mt. Kōya, this time establishing their headquarters on Mt. Negoro. This exodus marked the beginning of the Shingi Shingon School at Mt. Negoro, which was the center of Shingi Shingon until sacked by Hideyoshi Toyotomi in 1585.
During the initial stages of his predication in Japan, the Catholic missionary Francis Xavier 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 to Deusu from the Latin and Portuguese Deus. The monks also realized by that point that Xavier was preaching a rival religion.
The teachings of Shingon are based on esoteric Vajrayana texts, the Mahavairocana Sutra and the Vajrasekhara Sutra (Diamond Crown Sutra). These two mystical teachings are shown in the main two mandalas of Shingon, namely, the Womb Realm (Taizokai) mandala and the Diamond Realm (Kongo Kai) mandala. Vajrayana Buddhism is concerned with the ritual and meditative practices leading to enlightenment. According to Shingon, 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 properly training the body, speech, and mind, we can reclaim and liberate this enlightened capacity for the benefit of ourselves and others.
Kūkai also systematized and categorised the teachings he inherited into ten stages or levels of spiritual realisation. He wrote at length on the difference between exoteric (both mainstream Buddhism and Mahayana) and esoteric (Vajrayana) Buddhism. The differences between exoteric and esoteric can be summarised as:
Kūkai held, along with the Huayan (Jp. Kegon) school 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. The Dharmakaya is in fact a symbol for the true nature of things which is impermanent and empty of any essence. The teachings were passed from Mahavairocana.
In Shingon, Mahavairocana Tathagata 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 doctrine of Shingon, transmitted orally to initiates by the school's masters. 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 special emphasis on the Thirteen Buddhas, a grouping of various Buddhas and boddhisattvas:
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.
One feature that Shingon shares in common with the other surviving school of Esoteric Buddhism (Tendai) is the use of seed-syllables or bija (bīja) 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), the seed-syllable mandala or dharma-maṇḍala (法曼荼羅), the 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 the 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 that sound as a. 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 Mahāvairocanābhisaṃbodhi-sūtra) and Gosōjōjingan (五相成身觀, pañcābhisaṃbodhi "series of five meditations to attain Buddhahood" from the Sarvatathāgatatattvasaṃgraha).
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 (ie. the three mysteries). 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 (結縁潅頂), which is normally offered only at Mt. Koya, but is not required. In the case of disciples wishing to ordain as priests, the process is more complex and requires initiations in various mandalas, rituals and so on. In either case, the stress is on finding a qualified and willing mentor who will guide you through Shingon practice at a gradual pace.