During the Asuka period in which Buddhism was first transmitted to Japan, Shinto and Buddhism had not yet integrated with one another, but in the Heian period when Buddhism began to diffuse throughout the nation, a certain degree of friction arose between the new beliefs and the ancient Japanese Shinto religion and out of this rose a syncretism of Shinto and Buddhism in which Shinto kami were considered to be the temporary forms of Buddhist deities.
日本に仏教が伝来した飛鳥時代には、神道と仏教はまだ統合される事はなかったが、平安時代になり、仏教が一般にも浸透し始めると、日本古来の宗教である神道との軋轢が生じ、そこから神は仏の仮の姿であるとする神仏習合思想が生まれた。