Under what conditions do powerful ideological movements arise and transform politics? The Protestant Reformation changed the religious, social, and economic landscape of Europe. While the existing literature has focused on the mechanisms and institutions of its spread, this article argues that an important precondition for the spread of the Protestant Reformation was territorial fragmentation, and the political autonomy it offered local rulers. Local rulers could then protect the reform movement both from central authorities, and from local rivals. Where power was centralized, kings could more easily either adopt or defeat the new religion. Using a data set that includes measures of territorial fragmentation, I find that it is strongly associated with the rise and diffusion of the Protestant Reformation. Local political heterogeneity can thus protect and diffuse ideological innovations.