This one from eminent constitutional scholar and historian Michael McConnell (Stanford law school):
The idea of civil control over the Church was difficult to maintain during the days of a single universal Catholic Church with its headquarters in Rome. Church-state relations in those days almost inevitably consisted of conflict and negotiation between two institutionally separate authorities: the Church in Rome and the civil power, usually the monarch, in various nations of Europe. Neither could completely control the other. With the outbreak of the Protestant Reformation, however, governmental power over each national church became more feasible. Indeed, with the Peace of Augsburg in 1555, the principle that the prince had authority to determine the religion for his nation (“cuius regio, eius religio”) became a staple of international relations.
Michael W. McConnell, Establishment and Disestablishment at the Founding, Part I: Establishment of Religion, 44 Wm. & Mary L. Rev. 2105, 2191 (2003). — MOD