I think I've already stated my opinion on this, but numbers, at least sociologically and psychologically have weight to me.
Again, if a god claims that he is universal, and that his religion is universal, and that salvation passes ONLY through faith in him and his religion, if the total number of followers amount to 5, then I doubt it is true. Sure, this is extreme, but the fact that christianity has managed to become the most believed set of truths over the centuries, that the Bible is the most sold and read book in the whole world, to me has SOME weight to it. It doesn't prove shit but it proves to me that there's something functional in it.