The beginning of the twenty-first century has provided abundant evidence of the necessity to reexamine the relationship between Catholicism and the modern, global world. This book tries to proceed on this path with a focus on the meaning, legacy, and reception in today’s world of the ecclesiology of Vatican II, starting with Gaudium et Spes: “This council exhorts Christians, as citizens of two cities, to strive to discharge their earthly duties conscientiously and in response to the Gospel spirit.” Catholicism and Citizenship is a call for a rediscovery of the moral and political imagination of Vatican II for the Church and the world of our time.