Every popular religion is real. This is why (Just a thought I got):
When alot of people believe in one thing it must be real (Magick for instance, except only one person has to believe in it for it to work). I think it makes a thought-form and then it keeps collecting more and more power as more people believe in it. Then I guess if someone believes (When he prays to deity) then the thought-form sort of morphs and uses it's power to help it's prayed-for-cause. But if a certain God doesn't believe in what should be prayed for then all of the belief will stop the prayer from being fulfilled.