What was Martin Luther's main objection to the Catholic Church?

Martin Luther's main objection to the Catholic Church was its practice of selling indulgences, which he saw as a form of corruption and a distortion of true Christian teaching. Indulgences were a kind of certificate that the Church claimed could reduce the punishment for sins, either for the buyer or for souls in purgatory. Luther argued that salvation and forgiveness could not be bought but were instead gifts from God received through faith alone.

On October 31, 1517, Martin Luther famously nailed his Ninety-Five Theses to the door of the Castle Church in Wittenberg, Germany. These theses outlined his disagreements with the Catholic Church. Most notably, Thesis 86 questions why the pope, who is very rich, requires money from the faithful to build St. Peter's Basilica in Rome rather than paying for it himself.

Luther's actions and writings set off a chain of events that led to the Protestant Reformation, radically changing the religious, cultural, and political landscape of Europe.