This thesis examines early modern Japanese religions through a case study of the Ôyama cult, a cult of a sacred mountain in central Sagami Province. The thesis discusses the changes in the mountain's sacred geography from the medieval to the early modern periods; the establishment of its early modern Shingon institutions and of its professional innkeepers (oshi); the rituals that provided the Shingon clergy and the oshi with income; the pilgrimage to Ôyama in the context of the Kantô region and the development of Ôyama confraternities; the changes in Ôyama's pantheon due to the impacts of Shugendô, Shingon, Shirakawa Shintô, and nativism; and the fundamental changes in the Ôyama cult after the separation of Shintô and Buddhism, which led to the cult's development into Ôyama Keishin Kyôkai, a Sect Shintô church. To study these multifaceted aspects, this thesis uses an interdisciplinary approach that leads to conclusions that transcend the limitations of distinct fields. A study of the institutional, doctrinal, and ritual complexities of Ôyama illustrates the nature of Japanese religion, which defies neat categorization into distinct traditions. This approach allows us to view Tokugawa Buddhism as embedded in the context of Japanese religions in general. Instead of speaking of its intellectual stasis, moral and spiritual corruption, or its function as an instrument of the state—as historians of religions in the past have often done—we are able to cast Tokugawa Buddhism in more positive terms: during the Tokugawa period, Buddhism developed unprecedented institutional stability, penetrated into all levels of society, and remained a vital intellectual and spiritual force in society. The example of the Ôyama cult illustrates how deeply Buddhism was integrated into the religious landscape of Japan. A cult that initially survived on patronage by the bakufu came to depend almost exclusively on the donations of ordinary people—villagers, townspeople, and low-ranking officials. In its ordinariness, it occupied a middle ground that was neither at the center of political power nor in the margins of society. The Ôyama cult was not used by the bakufu to mobilize human resources or to suppress resistance to the regime, but it was also not a movement to subvert the existing social order. The Ôyama cult flourished—Ôyama literally became a Mountain of Great Prosperity—precisely because it was based on and upheld the social structures that shaped village and urban life. Hence it can serve as an example of mainstream Tokugawa religion and society.