This occurred as a result of the syncretism of Shinto and Buddhism in which the Japanese indigenous Shinto beliefs became mixed with the imported Buddhism (particularly Esoteric Buddhism) to create various religious belief such as Shugendo (Japanese mountain asceticism incorporating Shinto and Buddhist concepts).
背景には神仏習合の歴史があり、土着の神道と渡来して普及した仏教（特に密教）が混淆して修験道などの多様な信仰を生み出してきた。