Esotericism was central to both Buddhist traditions ascendant in the Heian period (794–1185): Shingon and Tendai. Shingon is an exclusively esoteric school founded in Japan by Kukai (774–835) whereas Tendai, founded in Japan by Saicho (766–835), includes esotericism as part of its grand synthesis of many Buddhist perspectives. Although several Buddhist schools had entered Japan by the end of the Nara period, for the most part none had developed into fully independent, religious establishments in their own right. That is: Shingon and Tendai were the first to flourish and develop as distinctively Japanese independent schools. Furthermore, both schools developed forms of Buddhist-Shinto synthesis. Both Shingon and Tendai arose about the time the capital moved from Nara to Kyoto (called “Heian” at the time). Their success was due in part to the fact that Buddhist esotericism shares central assumptions with early Japanese spirituality. With its centuries of doctrinal and practical development on the mainland, esoteric Buddhism was uniquely positioned to give early Shinto spirituality a full-blown philosophical justification, albeit admittedly a justification in Buddhist garb. Let us consider three points where the worldviews of ancient Shinto and esoteric Buddhism intersected—areas of similarity on which Shingon and Tendai Buddhism were able to capitalize.
First, for both early Shinto and Buddhist esotericism, the world was alive with spirituality, as there is no sharp divide between spirit and matter….
Second, Buddhism and early Shinto both stressed the purely mindful heart….
There is, as well, a third commonality: both esoteric Buddhism and early Shinto assume the sacred can be in the form of celestial deities (in Shinto, the kami deities; in esoteric Buddhism, the celestial buddhas and boddhisattvas)….
Not surprisingly, then, with syncretism as the norm, the term “Shinto” had no popular use in Japan until the development of state ideology in the middle of the nineteenth century. In that era, an essentialist Shinto spirituality was on the rise and the agenda was to separate “real” Shinto from its Buddhist “distortions.” Because of Buddhist-Shinto syncretism, however, it seems that for a thousand years of Japanese history most people did not ordinarily find it useful to distinguish “Shinto” from “Buddhist.” These people did all along refer to kami, of course, but they knew that on some level (perhaps understood only by intellectuals) kami were just alternative forms of buddhas.