The 'Diet of Worms'
In October, 1518, the Pope summoned Luther to attend a meeting of the Roman Empire's princes and other important people like lords, nobles etc. The main purpose was to convince Luther to regret what he's doing and apologise. Luther went, even though he wasn't sure if he would return back alive.
But of course, Luther stuck strongly to his heart and said that he would be glad to apologise if they could tell him what he had done wrong. After knowing that if he didn't apologise, he would be arrested, Luther escaped at night and returned back to Wittenberg.
On October 10, 1520, Luther recieved a Papal Bull (a official letter from the Pope) saying that he was going to be excommunicated if he didn't apologise within 60 days.
But Luther completely ignored that message and burned the Papal Bull in public. He was officially excommunicated by the Pope on January 3, 1521.
Pictures from: http://www.religionfacts.com/christianity/people/luther/bio.htm
