What was Martin Luther's primary objection regarding the Catholic Church?

Martin Luther's primary objection regarding the Catholic Church centered on the selling of indulgences. Indulgences were certificates that the Church sold, which purportedly granted remission of punishment for sins, both in this life and in purgatory. Luther saw this practice as a corruption of true Christian teaching and a significant moral failure, highlighting that salvation is achieved through faith alone rather than through purchasing indulgences. His 95 Theses, famously nailed to the church door in Wittenberg in 1517, specifically criticized the practice, arguing that it exploited the faithful and undermined the true essence of repentance and faith. This challenge to the authority and practices of the Church marked a pivotal moment in the Reformation, encouraging a shift towards personal faith and the questioning of established Church practices.