The big bang theory does not assume that there is no God. It is merely one of several explanations of the mechanics of cosmology.
Repeat: mechanics.... like working on cars.
So what does the theory assume about the Biblical creation? The obvious answer is that it assumes that the bible is wrong and silly. So what does the big bang theory say about the validity of the bible which states of itself that it is that Word of God? Obvously the theory rejects that though it may try to be polite about it. Either that or it implies that God didn't understand the Big Bang Theory or that God was not capable of explaining it.