One source of Martin Luther's conflict with the Catholic Church was his objection to the sale of indulgences. Indulgences were certificates sold by the Church that purported to reduce the amount of time a person would spend in purgatory after death. Luther argued that indulgences were a scam and that they undermined the true meaning of repentance and salvation. He also criticized the Church's practice of requiring priests to remain celibate, arguing that this was an unnatural and unnecessary restriction. Luther's criticisms of the Church eventually led to his excommunication in 1521 and the start of the Protestant Reformation.