This chapter reviews the existing literature on religious party change. The scholarly literature offers three major explanations for the divergence we observe in Catholic and Islamist parties’ trajectories and for why religious parties change, or moderate: religious, political, and institutional explanations. Despite major contributions to our understanding of religious parties, the literatures on Catholic and Islamist parties grew virtually independent of each other, focusing on entirely different sets of questions or factors that explain change in these parties. Building on the existing literature, this chapter lays out the theory developed in this book to explain religious party change. First, an overview of the political economic approach to the study of religion is presented; next, the chapter outlines the effect of institutions on political behavior, and in particular how religious institutions affect political behavior and religious parties. Finally, the major actors in the theory are analyzed before concluding with a comparative assessment of how Islamist and Catholic parties fit into the theory.

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.