Italianguy,
I think you didn't understand my post. The original riddle goes:
You're at a forked road, and don't know which road to take. There's two twinbrothers there , both know the directions you need, however you know that one of the identical twins always lies, while the other always tells the truth. you can't tell them apart and you only get one question.
In this version, the answer is to merely ask: "What road would your brother send me on if I asked him for directions?" Both twins will answer the opposite road to the one you're supposed to take.
The difference with this riddle? Since we are told that the angel who guards the door to heaven always tells the truth, and we are told that the angel who guards hell always lies, we can just ask any question. Any question that you already know the answer to will suffice. If you know the answer already, then by comparing their responses to the actual answer you can determine which on lies. Once we determined trough our question which of the two is the liar, then we know that the door that this liar is guarding, is the door to hell.
On another note, both Ardantio and Glo's answers were correct. why did you say they were close, but not right? Their answers totally work, even without the loophole I've just shown.
Glo, I guess what you mean is, if you're telling the truth, but think you are lying, you're still lying. So if the devil thinks he's lord, and wants to lie he'd say that God is lord instead. Although in fairness he could also lie by saying that yet somebody else is lord.