The Catholic Church became corrupted, due to its overgrown power. Because of the Pope's overspendings, the Pope started the Sales of Indulgences. The Sales of Indulgences was a scheme to earn more money. It made people spend money so that they/family members go into heaven. This upsetted a reformer, Martin Luther. That started the Reformation. So in summery, the Catholic Church gained to much power ( more than the nobility) and it wanted more money. BTW, this only a little bit of it.