What practice prompted Martin Luther to criticize the Catholic Church?

At first, the reason for criticizing the Catholic Church was selling indulgences. As this practice continued after the letter Martin Luther wrote to the Archbishop of Meinz, Luther tried to carry out reforms within the church, but he realized that this was impossible because of the attitude of the Pope. Namely, the Pope considered himself the head of the church, God is in heaven, the Pope is on earth, everyone who contradicted was a heretic, because the will of God is carried through the Pope. Luther's criticism continues to deal not only with indulgences, but also with the personality of the Pope and his like-minded people calling them false Christians, realizing that reforms in the church are impossible.