In this article, we argue that an analysis of the conflict around the nature and limits of European integration that arose between Catholic and Protestant Christian Democrats in the post-war era can shed new light on the expansionary dynamics that gradually came to characterize the project of European integration. Catholic Christian Democrats framed the unification of Europe as a relatively exclusionary cultural-civilizational endeavour, while Protestant Christian Democrats favoured a more inclusive conception of Europe that prioritised free trade over cultural homogeneity. Focusing specifically on Germany, we suggest that the eventual resolution of the intra-party struggle between the two camps in the early 1970s was a crucial enabler for including more and more countries into the European project. For it was only thereafter that Catholic Christian Democrats began supporting the expansion of European integration beyond the core Europe of the original Six, with geopolitical concerns gradually crowding out cultural ones.