This misunderstands the nature of a pre-condition. Consider the law of non-contradiction. It cannot be directly proven; but it is proven indirectly by the fact that every time a person makes a truth claim he is subject to the law.For a necessary precondition, the reasoning would have to work both ways. To claim that God is a necessary precondition for morality, you are claiming P->Q. If morality, then God. However, the claim needs substantiation so that you also have to show ~Q->~P: if there is no God, then there is no transcendent moral standard.