Martin Luther is known for his pivotal role in the Protestant Reformation, which began in the early 16th century. In 1517, Luther famously nailed his Ninety-Five Theses to the church door in Wittenberg, criticizing the Catholic Church's practices, particularly the sale of indulgences. His actions challenged the authority of the Pope and sparked widespread theological debate. Luther's insistence on justification by faith alone and the authority of Scripture laid the foundation for various Protestant denominations. His work not only transformed Christianity but also had significant cultural and political implications in Europe, leading to a decline in the Catholic Church's influence.