A major impetus for the activities of Reformation thinkers such as Luther was the corruption of the Catholic Church, whose political power had grown enormously through the medieval period. In his Ninety-Five Theses, Luther questions the practices of Catholic Church officials, as well as official and popular claims about the powers of the pope. In particular, Luther believes that some of those claims are not sound in view of fundamental principles of the Christian faith.
During the Middle Ages, successive popes declared more and more indulgences. Some of these indulgences were originally for those who supported charitable works undertaken by various groups under the Catholic Church. Church officials called pardoners were empowered to grant the indulgences to those who contributed money for the causes sponsored by the church. Gradually, the practice became corrupt. The Catholic Church came to view indulgences as a major source of money, and pardoners began to misuse their power for personal gain. Indulgences thereby changed from instruments to reward piety to commodities that were sold to believers.
Pardoners sought to extort as much money as they could from believers in return for indulgences. Further, pardoners deceived the common people who went to them, claiming that the indulgences included forgiveness for sins and not just exemption from punishment. (According to Christian theology, only God possesses the power to forgive sins. So the pope could only exempt sinners from penance; he could not forgive sins.) Others made even more extravagant claims such as forgiveness for all further sins. Some pardoners also forged indulgences. People also began to believe that copies of letters declaring indulgences from the past, and other physical artifacts, had the miraculous property of ensuring forgiveness for sin.
As a devout Christian, Martin Luther viewed sin and forgiveness as extremely important questions. Luther, who was himself a priest, noted how the practice of indulgences had degenerated into a commercial activity far removed from religious observance. He also noted how members of his flock had absorbed the erroneous beliefs that the pardoners spread. He concluded that corrupt church officials such as pardoners ascribed greater power to the pope than was possible within Christian theology. Luther pointed out that it was disrespectful to God to equate an exemption from the pope with God's forgiveness.
To summarize, the topic of indulgences was important to Luther's argument because indulgences were a major source of corruption within the church. Pardoners used them to mislead churchgoers on articles of faith and tempted them into sin by offering forgiveness in exchange for money.
