In this video from The Lord of the Rings, we have Gandalf (a magician) fighting a Balrog. You can tell from the video that Gandalf has had a long study and relationship with Belief. As the Balrog stands ready to cross the bridge, you can see the fear in Gandalf. He then goes to a magician's short-litany as he has trained himself to do. Listen to what he says:
"I am a servant of the secret fire,
wielder of the flame of Anor."
This line represents a strong connection Gandalf has to the belief that he is the wielder of Anor's flame, the secret fire.
You can read more about the flame of Anor here http://lotr.wikia.com/wiki/Flame_of_Anor
The short version is that the flame is synonymous with the light of the Sun. So here we have Gandalf reciting his belief or affirmation that he is a wielder of the light of the Sun. If you believe that beliefs have power, then it follows that Gandalf may have obtained his magical powers through practicing with this power. (Though the lore goes deeper than that, and other concepts aside from belief maybe involved.)Then he raises his staff, and a heavenly light, the flame of Anor, draws forth from it, and he surrounds himself with it.
"The dark fire shall not avail you, flame of Udûn!"
Expressing another belief that he will overcome the fires of hell (Udûn) in the Balrog. Good (Anor) triumphs over evil (Udûn).
The Balrog attacks, but is stopped by Gandalf's sunlight-shield.
"Go back to the shadow."
Gandalf, a Maia or angel of the ancient world, is ordering the Balrog, a fallen Maia associated with demonology and hell, to retreat. Fallen Maia have lesser rank on the heirarchy due to their disobedience or dissociation with Ilúvatar, the god of the LOTR creation myth. In this Gandalf is essentially giving a General's order to a Colonel.
We end with more Belief-magic and the famous line "You shall not pass!" (and perhaps an invoking of Gandalf's Generals' power over Colonels - in the book, his staff is even broken by this!)
+ Show Spoiler [text from LOTR] +At that moment Gandalf lifted his staff, and crying aloud he smote the bridge before him. The staff broke asunder and fell from his hand. A blinding sheet of white flame sprang up. The bridge cracked. Right at the Balrog's feet it broke, and the stone upon which it stood crashed into the gulf, while the rest remained, poised, quivering like a tongue of rock thrust out into emptiness.