What do the religious institutional structures look like in Catholicism and Sunni Islam? In order to trace how religious institutional structures may shape the early evolutions of religious parties, this chapter explores the historical development of structures of religious authority and how they looked when religious parties arose. The focus of the analysis for each religion varies to reflect the historical trajectories of their religious institutions and what such different trajectories mean for the analytical narrative of this book. The analysis proceeds in chronological order and begins with Catholicism to be followed by Sunni Islam. In the case of Catholicism, the focus is less on the evolution of religious institutions and more on how divergence from orthodoxy and orthopraxy are handled within the Catholic Church. As the religious authority in Catholicism, the Church’s handling of divergences is key to understanding how it dealt with the rise of Catholic parties. In the case of Sunni Islam, the focus remains on the historical evolution of religious authority, paying close attention to the parameters of major shifts in the eleventh and early twentieth centuries.