Heian Japan did not experience Buddhism and the native kami cults as rival religions to choose between; it developed a theological framework, honji suijaku, that explained them as two expressions of the same reality. Under this theory, a kami was understood as a suijaku, a trace or local manifestation, of an underlying Buddhist deity, its honji or original ground, so that worshipping a shrine's kami and worshipping the Buddhist figure behind it were understood as reaching the same object by different paths. A study of the doctrine describes it as a genuinely combinatory paradigm rather than a Buddhist absorption of native religion, since the identification ran in both directions and shrine and temple institutions frequently shared the same precincts and priesthoods.
The pilgrimage complex at Kumano, on the Kii peninsula south of the capital, became one of the clearest expressions of this combinatory religion in practice, drawing Heian aristocrats, including retired emperors, on a demanding overland pilgrimage to shrines simultaneously read as manifestations of Amida, Kannon and other Buddhist figures. A study of the Kumano pilgrimage describes a religious landscape organized around this fusion rather than around either tradition alone. The theological work of treating kami as Buddhist manifestations, alongside a doctrine of original enlightenment developed within Tendai holding that all beings already possess Buddha nature, gave later medieval Japanese Buddhism much of its working vocabulary.