This dissertation focuses on religious gatherings in communist Romania and the Soviet Union, 1943-1989. Church was one of the few opportunities for voluntary associational life and is invaluable for the study of power, ideology, and belonging in an everyday social setting. This project is based on archival documents and memoirs, uncovering how state officials and religious representatives struggled to establish religious practice that would be acceptable to all. Although ideologically atheist, state officials regarded some religious gatherings as acceptable and others unacceptable, but not due to utterances of beliefs or performance of traditional sacraments, but because of social aspects: how people related to one another, what kinds of people came, the settings of the gatherings, and affective characteristics like enthusiasm, engagement, and authenticity. Even though believers participated in religious gatherings for their own reasons, state officials policed them as contests for mobilization. This project compares the cases of the Romanian Orthodox Church and Reformed Church of the Transylvanian region of Romania and the Russian Orthodox Church and the Baptist Church in the Moscow region of the Soviet Union. Based on comparisons, the role of a Church's culture in shaping church-state relations becomes clear. Officials largely considered traditional Orthodox hierarchy and rituals as religiously unproblematic, but they underestimated the power of such features of Orthodoxy to endure and mobilize successive generations. The hierarchical nature of the Orthodox Churches did not preclude spirited negotiations over acceptable Orthodox religiosity, but non-conforming or innovating priests were marginalized relatively easily. Protestant Churches have had a more entrenched custom of decentralization in governance and Scriptural interpretation, factors which presented officials with difficulty in centralizing the management of such churches and which at times led to protracted interpersonal battles and inner-church divisions. One such case sparked the Romanian Revolution in 1989. Officials in Romania and the Soviet Union handled the problem of religion very similarly in defining the acceptable limits of religious activity in practice, but virulent attacks on religion in the Soviet Union prior to WWII made for a stronger lingering religious antagonism there after the War than in Romania, where Orthodoxy was at times incorporated into the state’s nationalist discourse.