Beginning from the observation that religious diversity today refers primarily to a distinct set of ‘religions’ and their subdivisions, this chapter traces the historical development of these understandings, their consequences, and their possible transformation in the current period. It begins by looking at certain concepts found in recent discussions of the ‘Axial Age’ thesis and suggests that the dominance of the religion/secular distinction is what characterizes modern religion that manifests itself as religions, not putatively Axial and pre-Axial distinctions like transcendent/immanent or sacred/profane. On the basis of a historical analysis, the argument proceeds to show how, beginning during the later Middle Ages, the development of institutional system differentiation in European society and the subsequent appropriation of this development in the rest of the world were the basis for the institutionalization of this modern religion and its religions. This prevailing pattern is now under some challenge, as alternate ways for forming the ‘religious’ become more prominent, including on the basis of different concepts (like spirituality and culture) and different distinctions, including transcendent/immanent and sacred/profane applied outside the framework of religious/secular. The chapter concludes with an illustration using results from research on the religion of second generation young adults in Canada.