This is what my script, Far End of the Black, was all about: faith vs. doubt, but also presented in terms of light vs. dark - hence the title meant to be taken literally and metaphorically as the apocalyptic land that is needed to be crossed is referred to as "The Black". As bleak as the subject matter is, it's all about finding "light" and being "enlightened" after formulating arguments for and against both faith and doubt wrapped up in a sci-fi/horror story - much like Shawshank Redemption uses hope and doubt. Anyone who's analyzed The Exorcist with a keen eye also realizes that story is about doubt/faith which is why Father Karras is really the main character.

TBBB, that's a great point re The Exorcist. As much as Regan's plight dominates the story, the real emotional crux is Father Karras, a priest who is doubting his faith. Hence that key question: Why does THIS story HAVE to happen to THIS character at THIS time? As far as Karras is concerned, he is at a pivot point existentially. The very real presence of Satan inside Regan is a visceral experience as compared to his conceptual form of doubt. Thus when he shouts over and over again, "The power of Christ compels you," he is FORCED to believe something even if his intellect says otherwise. Really, the dynamic tension between faith/doubt exists probably at the base of every story featuring a Protagonist transformation arc because the character changes from an Old Self into a New Self, and that requires faith to make that shift as well as concomitant doubts to overcome.

Yes, Karras is driven to find a psychological explanation at first for Regan's malady. I always found that to be an interesting contrast: a priest/father who's searching for a psychological issue, but nevertheless, it works out the way many great stories do: the main character has to overcome his own personal problem to be able to solve the problem of the story (often the two are related in the same exact story beat). And I agree, I think that dynamic tension is found at the base of many, if not every, story as we often see a character take/make that "leap of faith" in the moment of crisis at the climax.