Religion, that viable relationship between God and man, was a prime preoccupation of the medieval man, the reason for his existence, the central fact that dominated his whole life. There existed a basic agreement about God and how He should be worshiped. Europe was made up of a community of believers professing the same faith that, it was felt, was essential not only for the ecclesial community but for the body politic as well. To contemporaries the association and intermingling of the religious community and the civil and social world was highly desirable, indeed the normal way of life; this was the way things ought to be, had to be. And since unity of faith played such a central role in all facets of life, a heretic disassociated himself not only from the religious community but from the political and social community as well. He had declared himself a rebel!
This policy of basing political and social stability on the unity of faith characterized all societies of antiquity down through the Middle Ages – indeed too much so, for the pharaoh, king, emperor, etc. at times felt it necessary to declare himself a god in order to verify, legitimatize his authority to rule. This is the cutting edge of the distinction drawn by Christ: “Render unto Caesar the things that are Caesar’s, and to God the things that are God’s. “(Matthew XXII,21).
Albert Shannon, The Medieval Inquisition (1991)