Most significant of the early writers in our period is Marsilius of Padua, who laid out a thoroughgoing basis for denying political power to the pope and claiming it for the emperor instead. French and English writers contributed to the dialogue over the course of the 14th century, with Wycliff being one of the more important.
The Great Schism catalyzed the discussion, rendering it urgent. Those who argued that some sort of representative body of Christians was in fact superior to the pope in matters not only of faith but even of administration, these are grouped together by m odern historians under the heading of "Conciliarists" -- which is to say, they argued that a General Concil of the Church was the supreme body in Christendom.
Once papal supremacy in temporal matters had been effectively denied, the way was open for successors to Marsilius to argue that every king was supreme within his own kingdom. That, in turn, opened the very tricky question of the basis for that supremacy . By what right did a king rule?
The relative position of king and pope was more or less settled by the second half of the 15th century. When Luther came along and shattered the religious unity of Europe, the door was opened for the nation-state to assume pride of place in Western Civil ization. I won't spend much time on that development, for we have a separate course on the Reformation, but that event is the pivotal point in the end of Christendom and the invention of Europe. What we will concentrate on here are the developments ante cedent to that.