In Europe, the nineteenth and twentieth centuries are characterised not only by processes of secularisation, but also by religious reactions to the perceived secularisation of society. In the Catholic Church, the rise of active congregations constitutes a prominent part of this reaction. With their work "in the world", the members of the active congregations (brothers and sisters) had to represent and disseminate religious values in a modernising and secularising world. In the latter part of the nineteenth century, many of them became active in the field of education. Until well into the twentieth century, teaching sisters and teaching brothers were a customary feature in the system of Catholic education. This article provides a theory-driven historical-sociological and historical-educational analysis of this form of religious activism in the field of education. Overall, its focus is on mechanisms of social differentiation and inclusion in modern society at large, and on the opportunities and problems they create for fields such as religion and education. Empirically, the focus is on the rise and fall of the congregations of teaching brothers and teaching sisters in the Low Countries, especially Belgium.