As fun, transgressive, and quick-to-offend as horror can be, it is often quite conservative at heart. Just think of the means Van Helsing employs to give Dracula the willies: the crucifix. Christianity is often used as a simplistic symbol of good in horror--the supernatural cure for supernatural evil.
However, religion's role can be more complex, or at least less direct, than this, as LMK Sheppard argues in her new book Faith Horror: Cinematic Visions of Satanism, Paganism and Witchcraft, 1966-1978. While the surface conflict of many horror movies appears to be a matter of good/christianity vs. evil/satanism-paganism-witchcraft, the core conflict may actually be one of faith vs. commerce (Rosemary's Baby) or predestination vs. free will (The Omen). In the case of The Wicker Man, that conflict might not actually be between the film's two obvious opponents-- christian cop Sgt. Howie and the pagans of Summerisle--but between the faith of both of those adversaries vs. the secular reality that leaves all of them clearly screwed at the end of the picture.
I usually avoid academic studies like Faith Horror because I'm no longer in college and I often find this kind of analysis to be merely theoretical--it rarely deepens my appreciation for the topic discussed or convinces me that the author's theory is truly essential for a meaningful understanding of that topic. While this was the case with several of the chapters in Faith Horror, chapters such as the ones on Rosemary's Baby and The Wicker Man provided enough snacks for thought that I found the overall discussion worthwhile. A quotation from a New York Times article by Leonard Wolf that Sheppard quotes in her book's introduction sums up why I, an absolutely devoted non-believer, am so fascinated with all of horror's supernatural hogwash better than anything I'd ever read. The book also contains the best typo I've ever read, one that makes Rowan Morrison, the missing girl of The Wicker Man, seem like some sort of unholy hybrid beast born in the Joke Wall from Laugh-In.