What if religion was taken more seriously during the Iranian revolution in 1979? There are scholars who argue that it would have made a difference in international relations. These so-called ‘religionists’ argue that religion has unfortunately been neglected by International Relations. In their view, religion has never disappeared and it could even be argued that religion has seen a resurgence since the late 1960s. Why, therefore, has religion not been incorporated into theories of international relations? This book provides a critical reconstruction of the position of these religionists. It investigates how Hans Morgenthau’s classical realism and Kenneth Waltz’s neorealism deal with religion. Do these theories indeed ignore religion? Do they ignore it for the reasons put forward by the religionists? The conclusion is that the religionists quite convincingly argue for the importance and relevance of religion in international relations. However, the reasons given do not do justice to Morgenthau's classical realism and Waltz's neorealism, as they overlook the theological inspiration (Augustine and Niebuhr) of political realism and the importance of theorizing. As a result of an evaluation of both positions, this book presents an alternative approach inspired by the Amsterdam School of Philosophy, called a new Christian political realism. It incorporates the theological inspiration of political realism and the necessity of theorizing while doing justice to the relevance and manifold manifestations of religion in international relations.