Atheists often dispara theists by equating belief in God with belief in Santa Claus - both, say the atheists, are childish beliefs that should be abandoned. The theists usually reply that their God is far more subtle and sophisticated than a jolly red-dressed man flying around on a sledge, so the cases aren't comparable. But aren't they?
Why do we reject the Santa Hypothesis? There are two key reasons. One is that we caught our father filling the socks, not Santa. A little reflection reveals that this is the case for all of Santa's miracles - every sock ever filled, every reindeer sled-location reported by NORAD, every child molested by an old drunk man in a red suit - all a sham, a lie, errors and deceit.
It is this reasoning that the theists usually object to - their Santa, they say, is more subtle. He doesn't actually put money in people's socks, you know. No, he only did that once, a long time ago, and you can't prove that he didn't, now can you?
Well no, but that does not make it plausible. The stealthy, hidden Santa has not been has been reduced to intervening only when no one is looking very hard and that can only be seen if you squint and look sideways. This is absurd. The only kind of God left here with any intellectual respect is the Deist Deism God; positing a "hidden" god lurking in the cracks of our knowledge is just too embarassing. This God then, at least is left unharmed by the Santa argument. Right? Wrong.
There is a second reason why we reject the Santa Hypothesis. Long before we conduct double-blind experiments to monitor the filling of socks, we would reject th hypothesis out of hand because it doesn't fit into how the universe works. In the real-world, jolly old men don't run industrial toy-making complexes in the North Pole, and reindeers don't fly. We know something about reindeers, and drunks, and this just doesn't fit.
The same can be said about God. We know something about minds, about how minds were forged by evolution throguh the ages and how they are very physical, tangible, things. A non-physical mind, a mind that hasn't been borne by evolution yet shares human's intelligence and will, a personal God that can hear and understand or even just know... all of this just ignores what we actually know about actual brains. Real brains don't work like that. The universe is just not personal, it follows the laws of physics in a more uniform and impersonal manner. The universe doesn't revolve around us nor cares about us, its far grander than that. Things just don't work that way. It's childish to believe that they do, it's childish to believe the universe behaves in ways that contradict our scientific, educated knowledge about how the world works and how it's made up.
Is it possible that Santa exists? Sure. Perhaps one day a series of brilliant experiments will demonstrate the existence of Santa. Until that day, believing that there is a Santa - even a hidden one - is absurd. Believing that there is a God - even a hidden one - is just as absurd. Adults should believe the existence works like our methdological investigations, science, say it does, and that it contains the sorts of things that these investigations reveal. Flights of fancy that do not appreciate the full import or what it means to be a person, to fly, to deliver gifts, and so on should be left to children.
For anyone educated in the way the world really works and what persons really are, belief in the God Hypothesis is just as childish as belief in the Santa Hypothesis.