I think what I'm trying to say is that there's sort of a fundamental disconnect at work in movies like these. You've got immortal souls, immortal beings, afterlives, places of eternal punishment, all of which should be governed by distinctly metaphysical rules. (I'm not going to debate what those rules are for fear of starting a religious war!) Suffice it to say that belief should be a very strong part of those rules, and it's not unreasonable to suggest that a demonic villain can be defeated by a sincere and abiding belief in Jesus, Allah, or whoever. (It may be unreasonable to show it, given religious differences these days, but that's another story!)
I think you're confusing magical thinking with religious thinking. The two are not the same. These types of films take place in a magical universe, which is quite different from a religious universe.
Magic is essentially a form of technology--the use of metaphysical forces to affect the physical world. And the assumption underlying all magic is that the metaphysical world is governed by laws, the same as the physical world: as below, so above. The magician does not entreat metaphysical forces to come to his aid, the way a religious person does: he commands
them to do his bidding, in the same way that scientists and engineers mix chemicals or smash atoms.
Belief doesn't enter into it. The magician does not merely believe--he knows
. Indeed, he knows things that man was not meant to know. In such a world, arguing that a sincere belief in a deity should allow you to overcome demonic forces is like arguing that a sincere belief in being rescued should save you from drowning.
Take The Exorcist
, for example. The exorcism shown in this film, according to the Roman Rite, is a composite religious-magical ritual. The exorcists pray and entreat God to save His servant, and use His overwhelming power to drive the demon from Regan McNeill's body. But they also use various magical incantations and gestures which are intended to invoke that divine power automatically. "The power of Christ compels you!" they cry.
In a monotheistic religious universe, there is no possibility of the conflict that is central to drama: indeed, in theory, there is no conflict
at all. Everything happens according to God's will and pleasure. God will either intervene, or He won't, for His own ineffable and incomprehensible reasons. The protagonist's prayers will either be answered, or they won't. If they're answered, the protagonist will succeed: if they aren't, he won't.
In a magical universe, by contrast, God is remote, and metaphysical conflict is just as possible as physical conflict. Those who understand the laws of the supernatural world can manipulate it for their own ends, and use it to their advantage, just like those who understand the laws of the natural world. Solve the puzzle box, for example, and summon the Cenobites--if you dare. Draw a pentagram on the floor, take off your clothes, recite an incantation in old French, and the Devil will appear. Say "Beetlejuice" three times, and he will appear or disappear. And so on.