The truth of justification by faith alone became fixed in the heart of Martin Luther, and as he studied the books of Psalms, Romans, and Galatians, he was led to see more and more what this truth meant. This was no small doctrine. The whole understanding of Scripture is unlocked with this key. The Holy Spirit led Luther to use it and open the door.
Yet there was a problem. The Church of Rome, the only Christian church at that time, held no such key. In fact, she firmly shut that door and threw the key away. The Catholic Church offered indulgences to the people instead. Indulgences were a kind of forgiveness, or justification, one could earn by being sorry enough for sin and confessing enough, visiting enough relics, and giving enough money to the Church. What was enough? As the popes and bishops needed more money to build their luxurious palaces and churches, or to buy their way into positions of authority, the price of indulgences went up. Being sorry for sin wasn’t even necessary any more, just as long as the people gave a lot of money!
One of the most bold and outspoken preachers—or sellers—of indulgences was named Tetzel. He came very near the borders of Wittenberg, telling the people they would be forgiven as soon as they contributed their money. This was his slogan: “As soon as the coin in the coffer rings, the soul from purgatory springs.”
Purgatory is a place, so the Church of Rome teaches, that is like hell, but is where all people go for a time after they die, even if they are saved. This, too, is false. There is no such thing as purgatory. There is no such thing as indulgences. There is no such thing as buying forgiveness.
Luther heard about Tetzel. Some of his own flock from Wittenberg had bought some of the forgivenesses he sold. This was too close to home. That indulgences were a lie became clearer to Luther, and that he must do something about it became clearer, too.
He would post a set of theses, or in this case problems that he had with indulgences, on the door of the Castle Church in Wittenberg. This was common practice. Often professors from the university would put an item for discussion on the door there. It was a convenient place for fellow scholars to notice it. Yes, Luther would protest the sale of indulgences on the door, and maybe something would be done about this shameful practice.