Scholars of International Politics have recently come to grip with the global resurgence of religion. Since the inception of the field of International Politics, the secularisation thesis had been taken for granted and religion dismissed as unimportant. But in line with the current transformation affecting societies worldwide as well as with the re-consideration of the secularisation thesis by Sociologists, new resources must be developed within IR to better understand current events. While theories and concepts have been developed within Sociology and (...) the Political Sciences, no such tools are available in International Politics. Thus, this article provides a tentative theory of secularisation drawing on resources endogenous to the field. Drawing on recent advances in the broadly Constructivist tradition, this article re-interprets secularisation as a protracted international crisis of legitimacy. (shrink)