What practice was criticized by Martin Luther in his Theses?

The practice criticized by Martin Luther in his Theses was the sale of indulgences. Luther argued that the Church had strayed from its spiritual mission by monetizing forgiveness and salvation. Indulgences were certificates sold by the Church that supposedly absolved individuals of their sins or reduced their time in purgatory. Luther contested the notion that salvation could be purchased and emphasized that faith alone, not financial contributions, was the key to receiving God’s grace.

This critique was pivotal in sparking the Protestant Reformation because it challenged the authority and practices of the Catholic Church, leading many to question the integrity of the Church's financial operations and its spiritual commitments.