When the Protestant Reformation emerged in Germany, the Pope was the most powerful man in western Europe. He was the undisputed head of a church that claimed complete spiritual authority over the west, and was considered to be the living representative of Christ on earth. Not surprisingly, many popes were corrupted by this power and prestige, and by the sixteenth century, the church was ripe for reform.
Within this atmosphere, the critique raised by Martin Luther was particularly devastating to the church. Luther argued for justification by faith alone, rather than by works, and while the main thrust of his argument was aimed at the sale of indulgences by church officials, it had serious implications for church authority.
In questioning the legitimacy of indulgences, Luther proposed in his famous 95 Theses that Scripture alone should be considered as the ultimate authority for religious truth. Because he could find no scriptural basis for indulgences (and many other Church practices) Luther held they were illegitimate. Word spread rapidly, causing a serious problem for Pope Leo X. Not only was Luther threatening a major source of income for the church, as indulgences were, but he was calling the authority of the Pope himself into question by suggesting that he had no authority to endorse indulgences. As Luther expanded his critique in public disputations with Church officials, he went on to deny the authority of councils, and, by implication, the entire ecclesiastical structure of the Church itself. Clearly this represented a serious challenge to papal authority.