Indulgences were a practice in which the Catholic Church sold certificates that would reduce the amount of time a person would spend in purgatory after death. Luther argued that indulgences were a form of simony (the buying and selling of spiritual offices) and that they undermined the central message of the gospel, which is that salvation is a free gift from God through faith in Jesus Christ.
Purgatory
Purgatory was the belief that after death, some people would go to a temporary state of punishment called purgatory before they could enter heaven. Luther argued that there was no biblical basis for purgatory and that it was a way for the church to control people through fear.
The authority of the pope
Luther argued that the pope had no special authority from God and that all Christians had equal access to God. He also rejected the idea that the pope could forgive sins or that he was the head of the Christian church.
The veneration of saints
Luther argued that the veneration of saints was a form of idolatry and that it diverted attention from Jesus Christ, the only mediator between God and humanity.
The sacraments
Luther argued that the sacraments (such as baptism and communion) were not necessary for salvation and that they could not convey grace to a person. He believed that the only thing necessary for salvation was faith in Jesus Christ.
The role of the clergy
Luther argued that the clergy should not be a separate class from the laity and that all Christians were called to serve God in their daily lives. He also rejected the idea that the clergy had any special power or authority over the laity.
The role of the Bible
Luther argued that the Bible was the only source of authority in matters of faith and practice. He rejected the idea that the church had any authority to interpret or add to the Bible.