What was Martin Luther's main complaint against the Catholic Church?

Martin Luther's primary complaint against the Catholic Church centered on the sale of indulgences. In the early 16th century, indulgences were payments made to the Church that were said to reduce the punishment for sins, allowing individuals to buy their way into Heaven or lessen their time in Purgatory. Luther believed that this practice was corrupt and fundamentally misconstrued the teachings of Christianity, as salvation, he argued, could not be bought but was a gift from God through faith alone. This belief led him to write his famous Ninety-Five Theses in 1517, which criticized the sale of indulgences and called for a reform of the Church.

While concerns about clergy corruption, educational deficiencies, and papal authority were also prominent issues during the Reformation, Luther's initial challenge specifically targeted the financial abuses related to indulgences, which exemplified a broader criticism of the Church's practices and the commercialization of religious faith.