Martin Luther was a German monk and theologian who denounced the clergy as corrupt and urged reform of the Roman Catholic Church.
He wrote his Ninety Five Theses, in which he presented propositions on how to reform the establishment, and nailed them to the castle church of Wittenberg, Germany in 1517. This is now seen as the event that kickstarted the Protestant Reformation.
However, questioning Catholic teaching and criticism of papal policy was not permitted, and as a result, on January 3rd 1521, Pope Leo X issued the papal bull Decet Romanum Pontificem and excommunicated Martin Luther who was subsequently declared an outlaw at the Diet of Worms.
