Ryobu Shinto (両部神道)
"Ryobu Shinto" is a combination of Shinto and Buddhism based on interpretation of Shinto from a Buddhist Shingon Sect (esoteric Buddhism) standpoint.
In esoteric Buddhism, the universe is considered to be the manifestation of Dainichi nyorai (Mahavairocana). It is represented in the rules written in the Ryokai mandala Kongokai mandala centered around Dainichi nyorai and also in the Ryokai mandala Taizo mandara. The Butsu bosatsu drawn in the mandalas of the two realms, Vajradhatsu (Diamond realm) and Garbhadhatsu (Womb realm), were considered the true forms of Buddha, and god (Shinto) was interpreted as its manifested form.
The birth of Ryobu Shinto can be traced back to the introduction of Buddhism.
With the official introduction of Buddhism, Shinto, the faith in Japan since ancient times, was significantly challenged. Shinzen dokkyo (reading Buddhist sutras to god) was introduced, believing that Japanese gods were also seeking nirvana through Buddhism, Jingu-ji temples were built on the premises of shrines, and statues of god were made under the influence of Buddhism.
Around the end of the 8th century, Japanese gods were considered as one with Buddha, and the Honji-suijaku theory was born, which claims that the true form of Buddha, appeared temporarily in form of a god to save the people of Japan, and this later became the foundation for the idea of synchronizing Shinto and Buddhism.
In the latter half of the Heian period, Buddhist Shinto theory was developed by monks as a religious principle explaining Shinto theoretically. Among the two esoteric Buddhism that were mainstream in those days, the one adopting the teachings of the Tendai Sect was Sanno Shinto, and the one adopting the teachings of the Shingon Sect was Ryobu Shinto.
Both use the religious principles of esoteric Buddhism, mainstream at that time, to explain Oharae no kotoba and to describe the gods and enshrined deities of shrines that appeared in Japanese mythology.
Both were based on apocryphal books using Saicho, Kukai, etc. as a pretext for selectors, and were instructed as secrets of each shrine, or in some cases, became known among people through Shugendo (Japanese mountain asceticism-shamanism incorporating Shinto and Buddhist concepts). These were theorized in the Kamakura period, and later generated many Shinto theologies.
In Ryobu Shinto, Amaterasu Omikami, the enshrined deity of Ise-naiku (inner shrine), is the Dainichi nyorai of Garbhadhatsu, a Brahma-Deva, and a juniten (twelve deities), while Toyouke no Omikami of Ise-geku (outer shrine) is the Dainichi nyorai of Vajradhatsu, a Brahma-Deva, and a juniten. The Naiku and Geku of Ise Jingu Shrine are of the two realms, Vajradhatsu and Garbhadhatsu, and these two realms combined as one constitutes Ise Jingu Shrine, the manifestation of Dainichi nyorai.
(Nigu-ikko)
Additionally, it applied the trikaya (three bodies of the Buddha) of Buddhist Tathagata to the three gods in the Nihon Shoki (Chronicles of Japan), with Kuninotokotachi no mikoto as hosshin, Kuninosatsuchi no mikoto as hojin, and Toyokumo no mikoto as ojin, and considered these three combined gods to be the Dainichi nyorai, the principal object of worship in esoteric Buddhism.
In addition, it was considered that the seven generations of gods in the Kojiki (Records of Ancient Matters) was equal to Shichibutsutsukaige, and represents the stars of the Big Dipper. Izanagi and Izanami, Kami-sha and Shimo-sha of Suwa-jinja Shrine were also compared to the mandalas of the two realms.
Ryobu Shinto greatly influenced the development of Shinto theologies later on, and became the mainstream of Shugo Shinto theory in medieval times, generating many branches including Goryu and Sanrin.
However, by the end of Kamakura period to the period of the Northern and Southern Courts (Japan), as a counteraction to the Shinto theologies by monks, Ise Shinto and Yoshida Shinto, which advocate the Shinponbutsujaku setsu, which claims that god is the true form and Buddha is the temporary form, emerged, and became the mainstream religious principle of Shinto in the Edo period.
During the Meiji period, Ryobu Shinto was dealt a devastating blow by the separation of Buddhism and Shintoism in the first year of Meiji. This resulted in it losing its status as the mainstream Shinto doctrine.