Martin Luther objected to the selling of indulgences in his "95 Theses" because he felt that it was wrong to promise that souls would be relieved from purgatory based on a simple monetary contribution, and he also felt that the pope did not have the right to grant a pardon from God. An indulgence was a pardon for certain types of sins and the Catholic Church sold these indulgences to church members.
Luther did not believe that the pope could know whether or not the pardon would be granted and felt the church was essentially stealing from the people. Luther's "95 Theses" treatise was a way of organizing his thinking. It consisted of several statements and questions. Luther did not believe that many of the daily practices in the Catholic Church could be theologically justified. He caused quite an uproar without the Catholic Church's highest levels. In 1520, the Pope condemned Luther, but that simply made him more radical and fueled his continued explorations and writings.
Luther quickly became the leader and head of the Protestant Reformation that took place in 16th-century Europe. Luther was not the only person who broke away from the church, but he is one of the most famous. Luther was excommunicated from the Catholic Church, and he created his own church known as the Lutheran Church.