He does have a valid point there. If Whedon didn't want any mention of God or religion in his story, that's fine, he's free to ignore it. It just doesn't make sense to have Holy items have the power to repel or destroy vampires without acknowledging the Higher Power responsible for making them so. They got around something like this in the "Highlander" franchise by simply saying that holy ground, regardless of which deity it was dedicated to, was universally honored. Joss could have done something like that, or used the explanation that was used in "Fright Night," that it was actually the power of user's belief, not the actual artifact, that gave it the power to repel or harm vampires and demons. If they didn't have faith that it would work, then it wouldn't work for them. If they did believe, then it would, indeed, work.