I am an atheist, and hold great store in reason, and science. Not only do I not believe in god due to scientific reasons, but also simple logic. Apart from this simple logic all atheists will know of, what is there to refute something like God starting the inflation of the universe that we know of as the Big Bang himself - there seems to be a never ending loop, it's as if you could always arise at the conclusion, again disregarding the simple logic of a floating man in the sky controlling everything being completely absurd, that God started whatever created the universe. Say we discover and prove everything that led up to the Big Bang, how do we know that a God didn't just start that himself? And if this is impossible to refute, then why are we atheists at all, why not simply agnostics? It seems that we'd have to settle on "it's impossible to prove that he exists or does not exist." Or perhaps must we settle on, this is an instance in which there was an effect, but no cause?
But of course if we delve even deeper, perhaps into even philosophical depths, we arrive at the question: From where did that creator arise? The logical answer would be: he always existed. And yet again, it seems you must settle on another instance in which there was an effect, but no cause.