In 1516, a man named Johann (Yo-Han) Tetzel visited Martin Luther's town of Wittenburg, Germany.  He had been sent there by the Pope, who was the leader of the Christian church.  Tetzel was in Wittenburg to sell indulgences - certificates that guaranteed people a place in heaven.  Since people at this time were very religious and wanted to go to heaven, they bought indulgences using whatever money they had, even if they were poor.  The church was going to use this money to build a giant new cathedral in Rome, Italy called St. Peter's.

Martin Luther despised the selling of indulgences.   He believed that no person could buy their way into heaven, and that only God could forgive sins.  Luther wrote several letters to the church complaining about indulgences, but Johann Tetzel continued to sell them.