Two Japanese monks who traveled to Tang China in the same embassy of 804 returned in the following years with the teachings that would organize Japanese Buddhism for the rest of the Heian Period and beyond. Saicho brought back the Tiantai teachings he had studied on Mount Tiantai and founded the Tendai school on Mount Hiei, overlooking the new capital at Kyoto, building a monastic center that held the Lotus Sutra as its central text while also incorporating esoteric ritual, precepts, and meditation into a single comprehensive system.
Kukai, traveling on the same mission, studied esoteric Buddhism in the Tang capital Chang'an and returned to found the Shingon school on Mount Koya, a tradition centered on ritual and symbolic transmission from master to disciple rather than on scriptural study alone. Both schools received imperial patronage and both became the training ground for most of the Buddhist movements that emerged in Japan over the following centuries, including several founders who trained at Mount Hiei before breaking away to found their own schools well after the Heian Period ended. A study of Saicho's founding of Tendai treats the Mount Hiei institution he built as the single most consequential religious foundation of the era, not for what it taught in isolation but for the range of later Japanese Buddhism that grew out of monks trained within it.