My dissertation attempts to explain the rapid disappearance of radical Catholic thought in postwar Europe, which represents the surprising and heretofore unexplained domestication of liberal modernity's most persistent and radical critics. This phenomenon can only be explained through a serious study of revolutionary Catholic thought in the interwar period. During those years of cultural, economic, and political crisis, intellectuals across the continent flocked to the Church and claimed that only a return to Europe's ancient faith could stave off disaster. Interwar Catholicism was a contradictory enterprise in that it championed both a communal return to orthodoxy and an individual, existential commitment to Christ. In the controversies within Catholicism that arose during the Spanish Civil War and the Second World War, Catholic intellectuals rejected the idea of a continent-wide return to the faith, and transformed the notion of individual authenticity into a defense of democracy, ecumenism, European federalism, and human rights. These same Catholic intellectuals rose to positions of great political, institutional, and ideological power in the postwar decades. Drawing on recent sociological and philosophical research into the nature of secularity, my dissertation argues that radical Catholicism did not disappear at all, but rather transformed and secularized. Postwar Europe was, despite appearances, a Catholic moment in European history, and the hegemonic ideology of postwar reconstruction can best be understood as a secularized form of revolutionary Catholicism.