What would you do if there were no God? Would you commit robbery, rape, and murder, or would you continue being a good and moral person? Either way the question is a debate stopper. If the answer is that you would soon turn to robbery, rape, or murder, then this is a moral indictment of your character, indicating you are not to be trusted because if, for any reason, you were to turn away from your belief in God, your true immoral nature would emerge. If the answer is that you would continue being good and moral, then apparently you can be good without God. QED.
From his book, The Science of Good and Evil.