The only reason to believe in such a being is if, for some reason, one chooses to see a book or preacher as a definitive authority. And this, of course, is not a good base from which to convince others of one's beliefs.
"I believe in God because the Bible tells us He exists".
"OK, but why would anyone believe in the Bible?"
The main reason that people profess and internalise such beliefs is social: if many important people around you hold some authority dear, then you must have a highly independent mindset to question their beliefs. Also, such questioning of the sacred beliefs of your people is not a good way to stay a member of said people.
I used to feel that the scientifically most well-founded position on the god issue was agnosticism: "we have no way of knowing". But I've already mentioned Ockham. Without his rule, that unnecessary hypotheses should be avoided, we wouldn't be able to do science at all. So I'm an atheist. Science is no different from our everyday way of perceiving the world, just more systematic. And in science or everyday life, let's just forget about fairytale characters.
In Sweden, this is largely a non-issue. Here, people with fervent religious beliefs are seen as slightly nuts and/or quaintly ethnic. But in the world's most powerful nation, it's of course a huge deal. President Bush II isn't afraid to say in public that he's acting on the orders of an invisible being. If our prime minister said that, we'd have him committed.
There's a lot of discussion in the US about whether scientists should come clean about atheism or keep quiet about it to avoid alienating believers. So just to lend them some friendly support, let me say that I am an atheist scientist and that I think atheism is the only reasonable position to take if you know anything about science. But then, being Swedish, I have absolutely nothing to lose saying that. It's almost as uncontroversial as saying that scientists should not condone infanticide.
[More blog entries about atheism, science, christianity; ateism, kristendom, vetenskap.]