It has been so long since I last played CoC, that I do not remember if there were ever any priest PCs, but I like the was some other games handle Faith, like in the World of Darkness, where it is your faith and belief that makes a vampire vulnerable to a holy symbol, and not the symbol itself. A person without faith will not hold the evil at bay just by waving a cross around. Though in WoD, I think the faith and belief the vampire had in his mortal days may also influence what effect a holy symbol had on them. I don't remember the details on that now. So for CoC, I could see a strong faith keeping lesser evils at bay, but stronger ones would just look at your priest and laugh. Or take the faith and holy symbols in D&D. Those are powered by the deities that actually exist, and for them to be powered that way in CoC, you would have to decide if the human gods are real or not.