When German monk Martin Luther nailed his 95 Theses to the door of All Saint's Church in Wittenberg on 31 October 1517, he started a religious revolution. The document was about the church's practice of selling indulgences - but Luther's protest would grow into the Protestant Reformation. Witness hears primary sources from the time, and speaks to historian Lyndal Roper.(Photo: A portrait of Martin Luther by Lucas Cranach the Elder on display at the German Historical Museum in Berlin, Germany (Sean Gallup/Getty Images)