GOOD POSTING (NOT THAT I AGREE WITH IT): John S. Wilkins at Evolving Thoughts:
In simpler terms, as the old saying has it, bald is not a hair colour. Not believing in some religion is not a religion. It may be that those who are either agnostic about Christianity, or atheist about it, have some other set of commitments that might qualify as a religion, but they do not need to, just in virtue of being a not-theist or a not-knower. So the choice is between believing in Christianity or not-believing in Christianity. It is not a case of commensurable religions, but a religion and no religion. This is the privative fallacy, from the old term for a lack of something.