The 1302 papal bull Unam Sanctam was the culmination of the medieval conflict between church and state and declared the unrestricted authority of the papacy over Christendom. The relation of the spiritual, or ecclesiastical, and secular, or temporal, authorities had been at issue in the Latin church since the eleventh century. It was then that Pope Gregory VII sought to reform the church with his cry of libertas ecclesiae-“freedom of the church”-from secular rule. A custom of secular rulers investing bishops with their episcopal regalia at installations had was the neuralgic point. The papacy believed feudal lords had undue influence on the property and local leadership of the church. Supported by the growing body of canon law, Rome began to argue that the pope as head of the church wielded two powers—one spiritual, one temporal—and so secular rulers themselves were under the authority of the pope. This claim provoked an ongoing controversy between popes and emperors that escalated over the next two centuries.
By the start of the fourteenth century, much of the tension had subsided until Pope Boniface VIII and Phillip IV of France rekindled it. Boniface at first opposed Philip’s renewed taxation of the clergy to support war with England, to which Phillip responded by withholding contributions to Rome. Boniface reconciled with him only to have Phillip reopen the wound by deposing a French bishop, which canon law strictly proscribed because clergy were under the jurisdiction of the church. Boniface composed Unam Sanctam in November 1302 as an answer. The bull restated the position of the earlier Gregorian reforms that ecclesiastical jurisdiction usurped temporal jurisdiction and since the pope was the head of the church that meant temporal rulers were subordinate to the pope as supreme ecclesiastical ruler. Though the bull was never intended to claim papal sovereignty over the secular French government, as Phillip claimed, it nonetheless did assert that all temporal powers and people were subject to papal authority. This enabled Phillip to use it as pretext for an offensive against Boniface. In the summer of 1303, while the pope was at his summer residence in Anagni, a band of mercenaries contracted by Phillip attacked the palace, imprisoned Boniface, and plundered all that was in the residence. The pope was said to be so physically and emotionally shocked by the assault that he died within weeks.
As a papal bull, Unam Sanctam was incorporated into the code of canon law and remained authoritative even in Luther’s day. As he was preparing for the Leipzig Debate in 1519, Luther studied at length what canon law stated about the papacy and on this basis he first suggested privately that the pope might be an antichrist. For Luther, the claim of the papacy to supremacy within the church and to authority over the secular realm departed from the rights of a bishop, led the people away from the content of the gospel, and created further obstacles to the reform of the church.