The Protestant Reformation was a 16th century movement aimed at reforming corruption within the Catholic Church. Martin Luther posted his 95 Theses criticizing indulgences and papal authority in 1517, which helped spark the Reformation. This led to the formation of Protestant religions separate from Catholicism and a split in Christianity. The Reformation spread ideas like "faith alone" and challenged the Church's authority, while the Counter-Reformation strengthened Catholic doctrines and launched the Inquisition in response.