This article looks at the intersection between Protestant theology and sociology in the construction of the modern concept of religion. Set against the theoretical background of the functional differentiation of modern society, it identifies the origin of this concept in the discursive 'scientification' of religion by the emerging disciplines of the humanities and social sciences. In taking the life and work of William Robertson Smith (1846-94) as an example, the article analyzes the transformation of some specific elements of liberal Protestant theology into a set of universal features that came to represent religion as a modern concept. In this way, it argues against confusing the modern concept of religion with a 'Christian model' as such, and also against rejecting the concept as a mere ideological tool of secularist ideologies.