Martin Luther's central criticism of the Catholic Church was its practice of indulgences, which he argued corrupted true Christian faith. In 1517, Luther famously nailed his 95 Theses to the Wittenberg Castle Church door, denouncing the Church's sale of indulgences as a means to reduce punishment for sins. He challenged the notion that salvation could be bought and emphasized the importance of faith and scripture as the sole sources of divine authority. Luther's criticisms sparked the Reformation, leading to the formation of various Protestant denominations and significant shifts in religious practices and beliefs across Europe.