The decline in numbers of religious in the West is discussed in numerous studies. While there is a consensus about the statistical reality of decreasing numbers, scholars disagree about the alleged reasons for this decline. This article maps the field and presents a survey of four categories of answers to the question of why religious life declined during the twentieth century. A distinction is made between theories that ascribe the decline to (1) historical, (2) societal, (3) ecclesial, and (4) theological reasons. The first category views the decline as part of a historical-cyclical pattern of growth and decline. The second encompasses explanations that focus on secularization, professionalization, or new societal opportunities for women. Thirdly, post-conciliar church-organizational reasons will be discussed. Finally, pre-conciliar theology is investigated as a potential reason for the decline. While none of the reasons discussed here can be excluded from at least contributing to the decline, we demonstrate that some authors are mistaken in their conclusions due to misinterpreting data in a way that obscures the possibility of an emerging decline before the statistics peak in 1965 (which marks the end of the Council). We also demonstrate how theology has been an underestimated but significant influence on the statistics of religious life.