This paper builds upon the theoretical work of Talcott Parsons and Niklas Luhmann and offers a critical reconstruction of their views on religion (Christianity) and secularization in the western world. It discusses the relation between the functional differentiation of modern society, the individualization of inclusion imperatives and the changing expectations regarding inclusion/exclusion in religious communication. From this perspective, it analyzes secularization in terms of perceived problems of inclusion in religious communication, and in terms of the reactions of Christian religions to these perceived problems. It thereby shows how the theories of Parsons and Luhmann are useful for empirical and historical research, and how they open ...