The sixteenth century was a period of intense questioning of religious authority in its many forms, including that of the Roman pope. Martin Luther first publicly denied the doctrine of papal ius divinum, or the teaching that the pope was Christ's divinely appointed successor, in his 1519 writing the Resolutio. Asserting that the papacy was a human institution, he argued from scripture, patristic sources, and historical evidence, the last time he would draw upon such as “consensus” of authorities before he would be pushed to deny the infallibility of the council at Leipzig. The Resolutio shows that Luther's denial of ius divinum was strongly motivated by pastoral factors, the belief that the pope had usurped the central place of Christ in the church, neglected his duty to communicate the Word to his flock, and established a tyranny over Christian souls. Though ius divinum did not have the status of officially defined dogma, Rome's defenders responded quickly and vehemently to Luther's challenge, a reaction that reveals the importance of the pope's divinely ordained status to Roman ecclesiology and sacramental doctrine. Thomist scholar and Luther's Catholic interrogator at Augsburg, Thomas de Vio Cajetan produced perhaps the ablest defense of ius divinum of his time, grounding his argument on the literal meaning of the key gospel passages under debate, Matthew 16:18-19 and John 21:15-17. Luther and Cajetan debated each other directly over the question of ius divinum ; this dissertation argues that another, slightly later voice participated in this discussion, after the lines separating Catholic from Protestant had been clearly drawn. The voice was that of Reginald Pole, noble Englishman who was exiled from his country to become Cardinal of the church, as well as a central figure among the spirituali, as the individuals in Italy who shared a devotion to reform based on individual reading of the Bible were known. Like the other spirituali (and like Luther for that matter), Pole believed reform was stimulated by contact with the Word, and thus had both an individual and an institutional component. This double aspect of reform is evident in Pole's work De summo Pontifice, which outlines a spiritual program for the pope to follow so that he may perform his role as Christ's Vicar. The understanding of the papacy as set forth in De summo Pontifice seeks to reconcile the fundamental beliefs of both sides of the debate over ius divinum, so that the pope is at once Christ's divinely appointed successor, but as such, his duty is essentially pastoral, to show Christ to his flock.