Hundreds of years before Martin Luther was born, the Catholic church had made up a place called purgatory.  Even if you had been forgiven for your sins, the church decided that faithful people had to spend time in this place before they could get to heaven.  The more sins you had committed, the longer you had to stay in purgatory.  

    Martin Luther was a priest in Germany when the Pope was having money trouble.  Pope Leo the Tenth (Leo X) had great ideas for building beautiful church buildings in Rome, but he was short on cash.  He came up with the idea of selling indulgences. A priest named Johan Tetzel was given the job of selling indulgences in Germany. Tetzel went to churches in Germany and as part of the church service told people that buying an indulgence would knock some time off their sentences in purgatory. Of course, the more money you gave to Tetzel, the less time you'd spend in purgatory. Tetzel even sold indulgences to people for their dead relatives!  Martin Luther knew that no one could buy their way into heaven, and he thought that the way the Pope and his priests were acting made it seem like they were just using God to build a church! 

     On October 31, 1517, Luther nailed 95 reasons why he thought what the church was doing was wrong to the door of the church in Wittenberg.  That began the Protestant Reformation.