Within Ryōbu Shintō, the Great Sun Sutra (Dainichi-kyō, Mahāvairocana Sūtra) serves as the central doctrinal key for uniting Shintō and Esoteric Buddhism. Dainichi Nyorai, the “Great Sun” of this scripture, is understood as the cosmic source and ultimate reality from which all phenomena arise. On this basis, Shintō kami, especially the sun goddess Amaterasu Ōmikami and other major deities, are interpreted as manifestations or local expressions of this universal Buddha. Worship directed toward the kami is thus re-read as, at a deeper level, veneration of Dainichi’s all-pervading presence. This scriptural vision allows the sun at the heart of Shintō myth to be embraced as a direct gateway to esoteric Buddhist enlightenment.

The sutra also undergirds the characteristic “two-fold” structure of Ryōbu Shintō. Its teachings on Dainichi Nyorai are closely tied to the mandala system, particularly the Womb Realm (Taizōkai) and Diamond Realm (Kongōkai). Ryōbu Shintō appropriates this dual-mandala framework to classify and interpret the kami, situating figures such as Amaterasu within a carefully ordered cosmology that mirrors esoteric Buddhist thought. In this way, shrine deities are not merely honored as independent powers, but are placed within a comprehensive vision of a single, universal Buddha-nature unfolding in multiple forms.

At the ritual and symbolic level, the Great Sun Sutra provides the foundation for integrating Shingon esoteric practices into the world of shrines and sacred landscapes. Mantras, mudrās, and visualizations associated with Dainichi Nyorai are applied to kami worship, so that shrine precincts, natural sites, and the visible world itself can be experienced as expressions of the Buddha’s dharma-body. The sutra’s emphasis on the omnipresence of this cosmic Buddha allows nature-based Shintō reverence and esoteric Buddhist metaphysics to meet on common ground. Through this scriptural lens, the honji suijaku understanding of kami as local manifestations of universal Buddhist deities gains a systematic, theological justification, and the boundary between “Buddha” and “kami” becomes a matter of perspective rather than ultimate division.