Pretty much the same thing as Kamog. I believed as a kid, just because that's what all the adults told me to believe. Then I realized there wasn't really anything that was supporting the belief other than the Bible (and that's rather dubious, circular support at that) and "intuitions". And for me, when I examined my intuitions, I came across the puzzling question "If God created us, who created God?". The only answer I've really ever seen for this boils down to "God was always here". But then, it seems to me, the entire objection that theists have to a more scientific worldview (eg. Big Bang, self-organizing chemical reactions which we like to call "life"
, etc.) -- mainly that the question of where everything came from stops before Big Bang -- still holds with the theist worldview. In other words, "What was there before the Big Bang?" is the direct analogue to "What created God?". So, it seems to me, anyone who is attempting to be rational and accepting scientific theories of origins, and then puts forth the answer of "God" as what came before the Big Bang, is really just adding a layer and replacing one unanswerable with another unanswerable.
And really, whenever you speculate about the past that far back in time, it really is nothing more than idle speculation with very no real direct evidence, and only some indirect and theoretical evidence supporting it. IIRC, the only evidence that science currently has for the Big Bang is the "static" of background radiation that fits the theoretical models given. It's already been established time and again that there is no way to prove either position, it just depends on the personality of each individual.