Why do religious politics thrive in some societies but not others? This paper explores the institutional foundations of this process in Indonesia, the world’s largest Muslim democracy. We show that a major Islamic institution, the waqf, fostered the entrenchment of political Islam at a critical historical juncture. In the early 1960s, rural elites transferred large amounts of land into waqf—a type of inalienable charitable trust—to avoid expropriation by the government as part of a major land reform effort. Although the land reform was later undone, the waqf properties remained. We show that greater intensity of the planned reform led to more prevalent waqf land and Islamic institutions endowed as such, including religious schools, which are strongholds of the Islamist movement. We identify lasting effects of the reform on electoral support for Islamist parties, preferences for religious candidates, and the adoption of Islamic legal regulations (sharia). Overall, the land reform contributed to the resilience and eventual rise of political Islam by helping to spread religious institutions, thereby solidifying the alliance between local elites and Islamist groups. These findings shed new light on how religious institutions may shape politics in modern democracies.