Final answer:
Luther protested the selling of indulgences, a practice where the Church issued pardons reducing time in purgatory for sins, in exchange for money. He believed that forgiveness could only come from God and that faith alone was needed for salvation, leading to the Ninety-five Theses and the Protestant Reformation.
Step-by-step explanation:
Martin Luther protested against the practice of selling indulgences, which was a practice where the Church sold pardons that released people from performing penalties for their sins. Specifically, these indulgences were supposed to reduce or even cancel the time a soul spent in purgatory before reaching heaven. Luther was opposed to this because he believed that salvation could only be granted by God and was attainable through faith alone, not by purchasing forgiveness. His objections to the sale of indulgences were one of the main points in his Ninety-five Theses, which he nailed to the door of the castle church at Wittenberg and sparked a major discussion that led to the Protestant Reformation.
Luther's challenge to the sale of indulgences came to a head when Johann Tetzel began selling them to finance the rebuilding of St. Peter's Basilica in Rome. Luther argued that the pope had no control over purgatory and criticized the Church's practice of essentially allowing the wealthy to buy their way into heaven, which he saw as contrary to Christian doctrine.