The union of Buddhism and Shinto was done on the basis of a Mahāyāna doctrine, honjisuijaku, which explains the relation of the Absolute Buddha to the Historical Buddha. Aided by this doctrine, the theory assumes that the Japanese kami are Buddhas/Bodhisattvas who reveal themselves for the sake of sentient beings.
"Shinbutsu-shugō." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 14, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/shinbutsu-shugo
"Shinbutsu-shugō." The Concise Oxford Dictionary of World Religions. . Retrieved August 14, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/shinbutsu-shugo