The DFG project Legions of the Pope. A Case Study in Social and Political Transformation studies a religious actor (papacy) and a religious practice (pilgrimage), and addresses political questions of legitimate actorhood and agency in world society and international relations. Interested in the shifting categories of nation states and world society, the project situates itself in the emerging field “Sociology of International Relations”. Therein the major focus is on the impact of the ongoing transformation processes of globalization and Europeanization on political power. Informed by a constructivist perspective, agency, practice and identity are seen as major factors that can explain structural order and Great Power changes that go beyond economic and military capacity but rest on the sources of legitimate actorhood. The question of the role of religion is linked to the wider issues of nationalism and belonging, and the question how a social practice can turn ideas into power.