Martin Luther, believed in demons but he believed in God more. In that great Hymn "A Mighty Fortress Is Our God" he writes:

And though this world, with devils filled,
should threaten to undo us,
We will not fear, for God hath willed
His truth to triumph through us:

The Prince of Darkness grim,
we tremble not for him;
His rage we can endure, for lo, his doom is sure,
One little word shall fell him.

That hymn, first published in 1529, has been called "the greatest hymn of the greatest man of the greatest period of German history." It has also been dubbed the "Battle Hymn of the Reformation" and with good reason. The Reformation touched off one of the most influential movements in world history. But before this famous Battle Hymn could be written Luther had to battle his personal demons and exorcize them from his own life. Luther felt utterly worthless and incapable of carrying the burdens of priesthood. On occasion Luther even flogged himself in an attempt to keep himself from sin.

He was often, he felt, pursued and tormented by Satan and his cohorts. Until one day, while reading Paul's letter to the Romans, he suddenly understood the meaning of God's grace and how it is appropriated by faith. In that moment he came to understand that he was justified before God through faith and not by his works.

You might say that after this experience Luther was no longer possessed by his demons, he was sitting upright, dressed, and in his right mind.