We have reached the bridge of Khazad-dum...possibly my dad's favorite part in the entire trilogy...and Gandalf's showdown with the Balrog. As he stands on the bridge, he speaks these lines:

"I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass. The dark fire will not avail you, flame of Udun. Go back to the Shadow! You cannot pass."

I find this moment interesting not only for its dramatic value, but because it finally puts a limit on the power of evil. Usually, Tolkien's "good guys" have to sneak around, to disguise themselves and beware of the Shadow that is ever creeping closer. Evil can operate openly, reaching up to the borders of the strongholds of good, while the forces of light are understated, working in secret and hoping for the day when they can work publicly. In this scene, though, the power of good deals a decisive blow to evil. The Balrog is barred from coming any closer. The power that Gandalf serves is greater than the power that he serves, and he cannot come any farther. He does catch Gandalf with his whip and pulling him down into the abyss, but as we all know, that's not the end of the story. :) Despite Gandalf's seeming defeat, this moment restricts the power of the forces of darkness. There's a potential here for an interesting parallel with Christian spiritual warfare...still working on that one.