Martin Luther had already been excommunicated when the emperor Charles V summoned him to the imperial diet meeting at Worms in April 1521, under a safe conduct that a good many people expected to be worth nothing, given what had happened to Jan Hus at Constance a century earlier. He was shown a pile of his own books and asked two questions: were they his, and would he recant what was in them. He acknowledged the books and asked for time to consider the second.
Returning the next day, he distinguished between his devotional writings, his attacks on the papacy and his polemics against individuals, declining to withdraw the substance of any of them, and finished by saying that unless he were convinced by the testimony of scripture or by evident reason, he could not and would not recant, because to act against conscience is neither safe nor honest. The words most often quoted afterwards, here I stand, I can do no other, do not appear in the earliest accounts and were probably added when the speech was printed.
The emperor issued the Edict of Worms, declaring Luther an outlaw whom anyone might kill without penalty and banning his writings across the empire. His own prince had him seized on the road and hidden in a castle, where he translated the New Testament into German in about eleven weeks. The edict was never enforced in much of the empire, and the failure to enforce it is why there was a Reformation rather than an episode.