Giovanni de Medici, the son of Lorenzo de Medici, was born in Florence in 1475. He was created a cardinal at the age of 13 and became Pope Leo X in 1513.

Leo was a patron of the arts and education and he founded a Greek college in Rome. He also began rebuilding St. Peter's Church. To raise the money for this project he sold documents called indulgences that pardoned people for the sins they had committed.

Martin Luther, a monk from Wittenburg, was very angry with Pope Leo X for raising money in this way. He believed that it was wrong for people to be able to buy forgiveness for sins they had committed. Luther decided to write down his views on the subject. He then nailed the paper to the door of the church in Wittenberg.

Leo ordered Luther to stop stirring up trouble. This attempt to keep Luther quiet had the opposite effect. Luther now started issuing statements about other issues. For example, at that time people believed that the Pope was infallible (incapable of error). However, Luther was convinced that Leo X was wrong to sell indulgences. Therefore, Luther argued, the Pope could not possibly be infallible.

Pope Leo X died in 1521.