What practice regarding the forgiveness of sins first drew Martin Luther's attention?

The practice that first drew Martin Luther's attention was the sales of indulgences. In the early 16th century, the Catholic Church offered indulgences as a way for individuals to reduce the amount of punishment one has to undergo for sins. These were essentially certificates that could be purchased, promising forgiveness for sins or the reduction of time in purgatory for oneself or deceased relatives. Luther was particularly troubled by this practice because he believed it undermined the true essence of repentance and salvation, which should be based on faith and God's grace rather than financial transactions.

Luther's objections to the sale of indulgences prompted him to write his famous Ninety-Five Theses in 1517, which criticized the Church's practices and sparked the Protestant Reformation. His stance on indulgences became a central issue in challenging the authority and practices of the Catholic Church, highlighting his belief in salvation through faith alone. This focus on the sales of indulgences not only defined his theological stance but also led to widespread discussions about church reform during that era.