The logic is that a Creator, if he exists in a logical world, requires his own Creator to have been created in the first place, that Creator requires his own creator, and so on and so forth back through infinite levels. Therefore a Creator is infinitely improbable, which, as far as logic is...
As a theoretical astro-physicist once said to me, if you ask what existed before the big bang then you don't understand your own question. You get the same answer if you ask what is outside the universe, or what is the universe expanding into.
God is just a more complicated way of answering 'I...