Kūkai and the Shingon school stand at the heart of the formation of Ryōbu Shintō, providing both its conceptual backbone and ritual texture. Kūkai articulated a vision in which the Shintō kami were understood as manifestations, or “traces,” of deeper Buddhist realities, especially the cosmic Buddhas and bodhisattvas. This interpretive key, often expressed through the logic of original ground and manifest traces, allowed Shintō deities to be read as expressions of a single esoteric cosmos rather than as separate, competing powers. In this way, Shintō myths and shrine cults could be folded into an overarching Buddhist soteriology without erasing their indigenous character.

The Shingon school then elaborated Kūkai’s insights into a more systematic synthesis that came to be known as Ryōbu Shintō. Drawing on its rich esoteric symbolism, Shingon mapped its dual mandalic vision of reality onto Shintō deities and sacred spaces, offering a twofold reading of the same kami and shrines. Shingon ritual technologies—mantras, mudrās, visualizations, and other esoteric rites—were applied directly to kami worship, so that venerating a shrine could simultaneously function as participation in an esoteric Buddhist ritual. In many cases, Shingon clergy worked in close association with shrines, producing syncretic texts that reinterpreted classical Shintō narratives through the lens of esoteric doctrine.

Through this process, Ryōbu Shintō emerged as a sophisticated religious synthesis rather than a simple blending of two traditions. The kami were honored as local, immediate presences while also being read as gateways to the vast mandalic universe of esoteric Buddhism. Shingon thought and practice thus offered a way to see the visible world of shrines, myths, and rituals as the symbolic surface of a deeper, all-encompassing Buddha realm. This vision allowed practitioners to move fluidly between Shintō and Buddhist modes of devotion, experiencing them as two coordinated expressions of a single sacred reality.