Cleisthenes’s democratic reforms after 508/7 had a huge impact upon many aspects of Athenian politics and society. However, religion in Athens has often been viewed as a matter quite separate from the sphere of politics, with the common view being that the democratic revolution hardly affected polis-level religion at all. Students of the great Geoffrey de Sainte Croix – notably Robert Parker and Emily Kearns – have spearheaded this view, arguing that any religious changes brought about by Cleisthenes were conservative changes, simply continuing existing trends in state religion. According to this view, religious changes seen in the fifth century BC were a result, not so much of democracy, but of a natural progression in religious practice. This paper directly challenges this view of Athenian religious history. I will demonstrate that the reforms of Cleisthenes led to changes to polis religion that can be described only as rapid and quite profound. In little more than fifty years, Athenian democracy demonstrably changed the traditional priesthood and cult personnel as well as the financing of, and the participation in, religious cults and festivals. It also introduced state-based control systems into deme cults, and democratic ideals into distinctly religious settings. The reforms of Cleisthenes were thus a watershed in the history of Athenian polis religion.