In this book chapter Casanova examines the role of religion as an "irritant" in on-going processes of (nominally) secular European integration. Picking up the popular description of Europeans "believing without belonging" (or "belonging without believing"), Casanova explores the ways in which the latent Christian identity of modern Europe has frustrated Europeans by rearing its head in the cases of EU expansion up to and possibly including Turkey, the interior Europeanization of (mostly Muslim) immigrants, the inclusion of staunchly Catholic Poland in the EU, and the debate over including a reference to or acknowledgement of Europe's Christian heritage in the preamble to the European Constitution. In all cases, religion has demonstrated its resilience and refusal to wither away entirely, even when both elite and popular opinion in Europe is oriented towards a secular cosmopolitan narrative of Europeanization. Casanova concludes with a stiff warning to Europe about the current limits of its integration processes both within ( vis-à-vis immigrants) and without (vis-à-vis Turkey), and warns that even secular-cosmopolitan expansion cannot extend across the globe. Borders will have to be drawn at some point, creating once again a "fortress Europe" -- secular or not. This chapter was published in Religion in an Expanding Europe (2006, eds. Timothy A. Byrnes and Peter J. Katzenstein).