For many Catholic women, the continued practice of religious faith was crucial to maintaining physical and mental equilibrium and resisting dehumanization in Nazi prisons and concentration camps. Yet such activities had to be conducted with the utmost secrecy. If caught, prisoners faced brutal punishments and even death. Separated from the religious institutions and support of their pre-carceral lives, these women adapted traditional devotional practices to their new situation. Some emerged as spiritual leaders in their cells or barracks, even acquiring nicknames such as "provost," "vicar," or "prelate." They led daily prayers and religious singing. They celebrated so-called dry Masses - a type of paraliturgy that included the prayers of the Mass and spiritual reception of Holy Communion. In some cases, they were also able to arrange the actual reception of sacraments. This paper explores the content and purposes of this underground religious activity. Participation in this type of activity provided a strong sense of solidarity, helped to build resilience, and was correlated with networks of mutual assistance that enhanced prisoners' chances of survival. My work is part of a recent trend that seeks to recover aspects of cultural life in the camps. The sources for this work include not only memoirs and testimonies, but also material culture created in the camps, such as prayer books, crucifixes, and rosaries.