We know there is no God in the same way that we know there are no square triangles.
Assume: the definition of God is an intelligent being who decides to begin the universe, including time.
A decision is a transition from indifference to will. Such transition may only take place within time. Thus, we ask the question when did God make the decision to begin time, before, after or during the actual commencement of time:
1. Since all decisions require time to already be in effect, God cannot make the decision until time has already commenced in operation and accordingly, he cannot make the decision before or during the actual commencement of time;
2. Since time was already in existence after time commenced, God cannot be said to create it.
Thus, the definition of God as presented CANNOT exist because, like the square triangle, the definition CANNOT be show to exist in reality and thus, there CANNOT be a God.
However, this proof only shows that an intelligent being could not create the universe. It does not prevent the universe being the result of natural causes, something without decision making ability or chosen will. Accordingly, as far as I see it and to the extent of the validity of the argument, which I cannot flaw myself, the argument about the existence of God is now over.