I think that one of the things that plays into whether you find movies like "Excorcist" or "Omen" frightening, is if you believe in the powers depicted in the movie. I find both movies utterly terrifying, but primarily because I believe in the existence of demons, and the potential existence of an "anti-christ" If those things aren't real to you, they really won't frighten you much in a movie.
I'm not religious, I think that does it.
I am also a nonbeliever, so perhaps there is some merit to the point. I'd have to think of other examples of religious-based horror and whether they scared me or not...
I was about to say something to the same effect. The fact that I a believer is why I was outright frightened by "The Exorcist".
The belief added to the air of possibility, which is what scares me more than anything else. Not that a film such as "Alien" isn't likewise effective, just not as much.
"It's not so much that it's 'shut down' as the government has been rebooted into safe mode."
- Tim and Daniel Shamblin