Ryōbu Shintō emerged in the Heian period as Buddhist monks, especially those of the Shingon school founded by Kūkai, began to integrate Shintō kami worship into an esoteric Buddhist framework. Drawing on Shingon’s sophisticated cosmology, they interpreted the myriad kami not as rivals to the Buddhas and bodhisattvas, but as their local manifestations, appearing in Japan to guide beings toward awakening. This interpretive move, articulated through the honji–suijaku paradigm, allowed Shintō deities to be seen as “trace forms” grounded in a deeper Buddhist reality, rather than as independent or competing powers. In this way, the world of the kami was not discarded but re-read as a symbolic and practical gateway into Buddhist truth.

The synthesis was not merely theoretical; it took shape in concrete institutions and ritual life. Temple–shrine complexes, or jingūji, arose where Buddhist temples were established alongside, or even within, existing shrines, and monks often served simultaneously as ritual specialists for both traditions. Within these shared spaces, Buddhist liturgies and Shintō ceremonies were performed in tandem, gradually forming a single religious field in which the two could no longer be neatly separated. Texts composed by learned monks reinterpreted Shintō myths through Shingon esoteric teachings, giving doctrinal depth to what was already happening on the ground. Over time, priestly lineages developed that combined Buddhist monastic training with inherited shrine ritual knowledge, further stabilizing this hybrid system.

Within this evolving landscape, the notion of “Ryōbu,” or “Two Parts,” came to name the dual-aspect character of the synthesis: Shintō and Buddhism were understood as two interrelated faces of the same sacred reality. The identification of specific kami with particular Buddhas and bodhisattvas, and the reading of shrine deities as expressions of universal Buddhist principles, gave practitioners a way to move fluidly between shrine and temple without feeling any inner contradiction. Ryōbu Shintō thus arose not from a single decree or moment, but from a sustained effort to see the native deities and imported Buddhist figures as mutually illuminating, each disclosing, in its own idiom, the one underlying ground of awakening.