Ozment, Steven E. The Age of Reform: 1250-1550 : An Intellectual and Religious History of Late Medieval and Reformation Europe. New Haven, Conn. ; London: Yale University Press, 1980. Kindle Electronic Edition.
Chapter 4, “The Ecclesiopolitical Traditions” pp. 135-181 Part 3, “Royal and Papal Apologists” pp. 144-155
All influential organizations have had publicists, call them what you may. Ozment notes a “revival of Roman law in the twelfth century” (Ozment 1980, 144) which argued for a political sovereign. This was used to erode the power of the pope and church. In addition, Aristotle’s Politics was published in Latin about 1260. Aristotle’s view of government asserted it as a good and natural thing, as against a view of government as a necessary evil. At the start of the 13th century France and England made a claim of the right to tax clergy. Pope Boniface VIII in 1296 threatened to excommunicate rulers who taxed clergy (Ibid., 145). By 1297, with a French ban on export of bullion, Boniface dropped his claim. King Philip IV continued to press for more control over the church. Boniface responded with his bull, Unam Sanctam in 1302 (Ibid., 145).
The debate continued on both sides as each sought to assert authority and control. The thrust of the papal argument was that of authority from God exercised over all, including emperors. The imperial argument was that of authority granted by subject peoples as a group to emperors who would exercise rule over others including popes.