The publication of Martin Luther’s Ninety-Five Theses in late 1517 initiated theological debates over the nature of grace and faith, true repentance, and authority in the Church. While Dr. Luther continued his teaching at the university in 
Wittenberg in 1518, he also engaged in polemical debate in Heidelberg among his fellow Observant Augustinians in April. In August 1518 he published the Explanations of the Ninety-Five Theses to defend himself against his theological critics and explain to Pope Leo X the meaning of his theses.
Dr. Luther received a summons from Pope Leo X to Rome to answer accusations of false teachings. This was the first step toward a potential trial for heresy. During 1518 the Imperial Diet (a large meeting of various rulers in Holy Roman Empire of the German Nation) was taking place in Augsburg. Holy Roman Emperor, Maximilian, sought to convince the German princes to choose his grandson, Charles to be his successor. Pope Leo wanted the German princes to approve a tax to fund a crusade against the Turks.
Luther’s prince, Frederick the Wise (Elector of Saxony) opposed both of these proposals. Frederick and his secretary, Georg Spalatin, convinced the pope to allow Luther to speak to a papal representative at Augsburg in lieu of traveling to Rome. Cardinal Thomas Cajetan was the papal legate at Augsburg. As a member of the Dominican Order, he had recently completed a commentary on Thomas Aquinas’ Summa Theologica (one of the most significant theological textbooks from the 13th century). Because of the importance of Frederick’s support, Cajetan’s task of fulfilling the papacy’s diplomatic goals became integrated with dealing with Luther’s teaching on indulgences and grace.
Martin Luther arrived in Augsburg at the end of the first week of October. Luther’s meetings with Cajetan took place from October 12-14, 1518 at the home of Fuggers, the same bankers who had lent Albrecht of Mainz the funds to pay the papal court for his elevation to the position of cardinal. Albrecht had promoted the preaching of indulgences in Germany to pay back the Fuggers.
While the meetings took place over three days, the discussion revolved around two main doctrinal issues: the treasury of the merits and justification. First, Cajetan referred to Pope Clement VI’s decree (entitled Unigenitus) from 1343. This document asserted that the pope had the power to apply the treasury of merits, earned by Christ, for the remission of the temporal penalty of sins through offering indulgences to the faithful. Second, Cajetan challenged Luther’s assertion that faith granted one certainty of salvation.
While Cajetan insisted that he would not debate Luther as an equal, he did allow Luther to produce a written statement on these issues on October 14. Luther pointed out that Clement VI’s decree did not even appear in all collections of canon law and it distorted Scripture’s teaching on the office of the keys. He argued that the pope was subject to Scripture and perhaps to a general council. Luther then turned toward faith and salvation. He argued this point more forcefully than the first because he thought it was more important. Citing Romans 1:17, he stated that men and women must have faith in God’s promises in order to be justified before God. He then cited numerous biblical examples of this teaching.
Not surprisingly, Cajetan rejected Luther’s statements and renewed his call for Luther’s recantation. While Luther remained in Augsburg until October 20, he did not meet with Cajetan again. When he departed in the middle of the night Luther’s standing in the church remained in jeopardy. Luther would not sway Pope Leo and his legate to his position. More importantly, would Frederick the Wise continue to protect him? Ironically, on October 31, 1518, Luther returned to Wittenberg exactly one year from the day he had mailed the Ninety-Five Theses to Archbishop Albrecht of Mainz. (Luther and 95 Theses)
Sources for this post:
Martin Brecht, Martin Luther: His Road to Reformation (1483-1521), trans. James L. Schaaf (Minneapolis: Fortress, 1985), 246-260.
Scott H. Hendrix, Martin Luther: Visionary and Reformer (New Haven: Yale, 2015), 72-75.
Lyndal Roper. Martin Luther: Renegade and Prophet (New York: Random House, 2017), 97-107.