Honji suijaku

The term honji suijaku or honchi suijaku (本地垂迹^?) in Japanese religious terminology refers to a theory widely accepted until the Meiji period according to which Indian Buddhist deities choose to appear in Japan as native kami to more easily convert and save the Japanese. The theory states that some kami (but not all) are local manifestations (the suijaku (垂迹^?), literally, a "trace") of Buddhist deities (the honji (本地^?), literally, "original ground"). The two entities form an indivisible whole called gongen and in theory should have equal standing, but this was not always the case. In the early Nara period, for example, the honji was considered more important and only later did the two come to be regarded as equals. During the late Kamakura period it was even proposed that the kami were the original deities and the buddhas their manifestations (see the Inverted honji suijaku section below).

The theory was never systematized but was nonetheless very pervasive and very influential. It is considered the keystone of the shinbutsu-shūgō (harmonization of Buddhist deities and Japanese kami) edifice.

Early Buddhist monks did not doubt the existence of kami but saw them as inferior to their buddhas. Hindu deities had had the same reception: They were thought of as non-illuminated and prisoners of samsara. Buddhist claims of superiority, however, encountered resistance; monks tried to overcome it by deliberately integrating kami in their system. Japanese Buddhists themselves wanted to somehow give the kami equal status. Several strategies to do this were developed and employed, and one of them was the honji suijaku theory.

...
Wikipedia