His strong step consisted in the making of a bonfire.

On December 10, 1520, as the students of the great University at Wittenburg came to the college, they found fastened to the walls a notice inviting them and the professors, and all who liked to come, to meet Martin Luther at the east gate of the college at nine o'clock the following morning.
Full of curiosity, they assembled in great numbers to find a bonfire, and Luther standing by it with a paper in his hand.

That paper was a letter from the Pope to Luther, telling him that if he did not recant from all he was teaching in less than sixty days, the Pope would give him over to Satan.

After reading the letter to the assembled crowd, Luther solemnly threw it into the flames and watched it burn to ashes, that all might see how little he cared for the Pope or his threats.

From that time there could be no more peace between Luther and Rome.
It was certainly a strong measure, and Luther owns that he had to make a great effort to force himself to take it.