Martin Luther was a German professor of theology, composer, priest and a figure in the Protestant Reformation which started in the 16th Century. He belived that you don't just pay the Church to get rid of sins, you had to have faith God to remove them.
The Catholic Church became corrupt and needed money which was gained by indulgences. Martin knew this was wrong so he decided to not stand by it.
Martin Luther tells people during this time of the wrong the Churches is doing by writing the 95 Theses and nails it to the front of the Church doors. Martin gets followers because the people realise what the church is doing is not acceptable.