Martin Luther was a well rounded guy. He was a professor of theology, composer, priest, monk, and a key figure in the protestant reformation. However, perhaps he was best known for his 95 Theses that he nailed to the door of the Wittenberg Castle church.
On October 31st of 1517 it is said that Martin Luther nailed the 95 Theses to the church door. They consisted of a list of questions and propositions for debate. This was a bold act of defiance meant to further the cause of the protestant reformation. After this the Catholic church was forever divided as Protestants did get their say in things as well.