The purpose of this study was to assess developmental and social determinants of the age at which children become aware that the social environment can be marked by categorization into religious groups and that those groups are associated with different religious beliefs. The results show that middle childhood is a critical period for this religious social categorization. Moreover, social factors play a role in the development. Religious categorization is likely to appear sooner in children attending heterogeneous schools than in those at homogeneous schools, and children from the minority religious group in the country understand religious categorization earlier than children from the majority group. However, no relation was found between the age at which religious categorization was understood and parents' religious socialization practices. This study is of both theoretical and practical interest: It complements what is already known about gender, race, and ethnic categorization by integrating developmental and social frameworks, and it can serve as a guideline for educational programs. (Contains 1 table.)