The Politics of Religious Party Change addresses the timely question of ideological change and secularization of religious political parties and asks, when and why do religious parties become less anti-system? In a comparative analysis, this book traces the striking similarities in the historical origins of Islamist and Catholic parties in the Middle East and Western Europe, chronicles their conflicts with existing religious authorities, and analyzes the subsequently divergent trajectories of Islamist and Catholic parties. Religious parties are embedded in distinct religious institutional structures that shape their actions as they chart their paths in electoral politics. Counterintuitively, the book finds that centralized and hierarchical religious authority structures – such as the Vatican – incentivize religious parties to move in a more pro-system, secular, and democratic direction. By contrast, less centralized religious authority structures such as in Sunni Islam create a more permissive environment for religious parties to operate as anti-system parties hostile to democracy and secularism.