The Reformation was a period of European history, usually thought to have been initiated after the publication of the Ninety-five Theses by Martin Luther. The Reformation was a schism in Christianity that led to the development of many different confessions (such as Calvinists, Lutherans, Protestants, etc.)
There were many causes for the Reformation. The selling of indulgences was a particularly strong one. The doctrine of the merits of the saints was also heavily criticised by Luther. He was also angered by the corruption and greed he saw within the Catholic Church.