Honchi-suijaku is one of the thoughts of Shinbutsu-shugo (the amalgamation of Buddhism with Shinto) that emerged during the era in which Buddhism flourished in Japan; it supports the view that yao yorozu no kami (eight million gods) are Gongens, the embodiment of various types of Buddhas (including Bosatsu (Bodhisattva) and Tenbu (Deva)), which appeared in the land that was Japan.
Honchi indicates the state of enlightenment, while Suijaku, which literally means "bringing down jaku (Ato)," indicates the appearance of Shinto and Buddhist deities. The ultimate Honchi is considered to be Hossin (the truth of the universe) and is therefore called Honchi-hossin. As the "gon" of Gongen means "provisional" or "temporary," as shown in "Gon-dainagon (an acting chief councilor of state), Gongen indicates a Buddha that has tentatively appeared in the form of a god.
The concept of Honchi originated from the inclusive nature of Buddhism, which encompassed a wide range of indigenous religions as it spread throughout the region. This is evidenced by the fact that most Buddhist Tenbu (Deva) gods originate from Hindu, India's indigenous religion. The thought also gave rise to a theory of esoteric Buddhism (of the later Mahayana Buddhism) that Kajishin like Fudo myo-o (Acala) is the embodiment of Dainichi nyorai (Vairocana).
On the other hand, the concept of Suijaku came from a Chinese character, 迹 (jaku), which first appeared in the Tenun chapter of "Soshi (Zhuang-zi)" or in its phrase of '所以迹' (the path of edification), and was later referred to by Guo Xiang of West Jin, the writer of "Zhuang-zi Commentary" who developed his own theory in the chapter of "Seong-wang (Daiseigaio)" and defined jaku as a royal reign, citing "所以迹" as a born saint.
It was Seng-zhao, of the late Qin, who first brought the concept to Buddhism. Seng-zhao cited, in his writing "Chuyuimakitsukyo (Vimalak?rti Sutra commentary)," the idea of "Honmatsu," which had been referred to by Wang Bi of Wei, and other scholars replaced the word "所以迹" with "本" (hon) and defined it (hon) as an unfathomable nirvana (enlightenment) achieved by Bodhisattva and "迹" (jaku) as a means to enlighten people, as used by Bodhisattva.
His theory, as endorsed by Hokekyo (Lotus Sutra), which supports Kuon jitsujo--a view that Buddha had been in nirvana long before he was born into this world--further influenced Zhiyi of the Chinese Tendai Sect, thus providing a basis for the development of Hokekyo's educational interpretations and the division of its scripture into "Jaku-mon" (the first half) and "Hon-mon" (the second half). This created a widely accepted view that Hon is the Buddha or enlightenment itself while Jaku is simply a means by which to save and enlighten people.
In Japan, as is evident in the confrontation between the Mononobe clan and the Soga clan during the Nara era, there has been a dividing line between Shinto and Buddhism since it was officially introduced to Japan. However, the line was gradually eroded as the Buddhists took the view that Shinto gods are a kind of lost, poor people, like the Tenbu (Deva) gods. They tried to raise their status to that of the Buddha by leaving copies of sutras at temples or doing Doso, officially acknowledging the gods' achievements and stating in oracles that they would no longer stay as a god thanks to Buddha's good deeds.
In the Tenmu period of the late seventh century, when the Emperor-centered political regime was put in place, the Shinto gods that had played important roles in building the country--including Amaterasu Omikami, the highest ranking Ujigami (local god) of all--were elevated in status to ethnic gods, and this encouraged Buddhists to give them higher ranking as a token of respect. Because these gods were considered to be members of the Goho zenjin, a group of gods who respected and guarded the Buddhist law, some were even granted Bosatsu-go (the title of Bodhisattva) from the end of the Nara era through the Heian era. A typical example of this is Hachimanshin, a famous ethnic god also known as Hachiman Daibosatsu.
However, some low-profile minor ethnic gods, like the spirits of nature and death, to whom the Honchi-suijaku theory was not applied, were treated differently. They are, for example, Jitsurui no kami (Jitsurui gods) or Jissha no kami (Jissha gods), as opposed to Gonsha no kami. To make their position clear, some Buddhists codified a warning that permitted people to respect Gongeshin but not Jitsurui no kami. This must have been an intentional effort to keep Buddhism separate from Shinto.
The theory of Honji-suijaku affected art and architecture, giving birth to Honchi-suijaku zuga (pictures) and Gongen-zukuri (buildings), and during the middle and end of the Kamakura era it affected literature as well, resulting in a series of works called Honchimono.
By the middle of the Kamakura era there appeared a group of people who insisted in the Shinpon butsujaku theory that Buddhas were in turn the embodiment of Shinto gods and that Shinto gods were the masters that Buddhas followed. This theory was originated by Shinto people who had complained about Buddhism, which had been dominant over Shinto, and wanted Shinto to achieve independent status from it. The Watarai clan, who had served as priests of Gegu (the outer shrine of Ise jingo) organized and re-edited myths and the records of Shinto ceremonies in order to compile the Shinto Gobusho (five-volume apologia of Shinto); their work laid the foundation for Ise Watarai Shinto.
Additionally, an attempt was made to theorize Shinto by borrowing the doctrine of the Tendai sect, which had adopted the reality-affirming Hongaku philosophy, and to re-edit several kinds of theory books by falsely attributing them to Kukai; but later these works were organized by Yukitada and Ieyuki WATARAI. From the period of the Northern and Southern Courts through the Muromachi era, the anti-Honchi-suijaku theory increasingly gained ground to the point where some monks of the Tendai sect supported it. Jihen, the author of "Kujihongi Gengi" and "Toyowashihara Shinpuwaki," converted to Shinto; and Ryohen of the Tendai sect, the author of "Jindai no maki shikenbun" and "Tenchi reiki kibunsho," supported his theory. Following these moves, Kanetomo YOSHIDA wrote the book titled "Yuiitsu shinto myoho yoshu (The Only Shinto Scripture)," making Jihen's theory more complete and comprehensive. However, new Buddhist schools that emerged during the Kamakura era supported the Honchi-suijaku theory as they were supposed to do.
Note that the anti-Honchi-suijaku theory was given its name by the scholars of the Showa era.
Suijakushin (the god of suijaku) and Honchibutu (the Buddha of honchi)
The Buddha, the identity of the gods, is called Honchibutsu. Buddhas that are forcibly linked to Shinto gods are different from religion to religion, faith to faith, temple to temple and shrine to shrine.
Although the most popular Buddhist title granted to Japanese gods is Bosatsu (Bodhisattva), some of them--like Hachiman Daibosatsu, whose Honchibutsu title is Amida nyorai (Amitabha)--carry a Honchibutsu title that differs from the corresponding Buddhist title.
The following shows some of the relationships between Suijakushin and Honchibutsu:
Amaterasu = Dainichi nyorai (Vairocana) + Juichimen Kannon Bosatsu
Hachimanshin = Amida nyorai (Amitabha) = Emperor Ojin
Kumano Gongen = Amida nyorai (Amitabha)
Oyama kui no kami = Amaterasu omikami = Dainichi nyorai (Vairocana)
Ichikishima-hime = Benzaiten (Sarasvati)
Atagoo Gongen = Akiba Gongen = Jizo Bosatsu (Jizo Bodhisattva)
Susanoo = Gozu Tenno
Okuninushi = Daikokuten (Mahakala)
Tosho Daigongen (Ieyasu TOKUGAWA) = Yakushi nyorai (Bhaisaya)
Matsuo Taisha Shrine = Yakushi nyorai (Bhaisaya)