The Reformers spilled much ink debating the legitimacy of the papacy with their Roman Catholic opponents.
Given the centrality of the papacy, that is not very surprising. Martin Luther began serious study of it in the early months of 1519. He soon became convinced that the authority claimed by the Pope was questionable. The Pope was not the head of the Church. Final authority lay in Scripture and duly-called councils.
Johann of Eck
Luther’s new view was soon put to the test in a debate with a leading Roman Catholic theologian named Johann Eck (1486–1543) at Leipzig in June and July of 1519. Eck, who took his surname from his place of birth in Germany, had a doctorate in theology and would write a major work on papal primacy in the year of his debate with Luther.