To show that the existence of God is necessary for ethics, I have heard a Christian say, "if God does not exist, then I have no reason to live morally - I might as well lie, murder, rape, and pillage."
In response I have heard the atheist say, "I am sorry that you are so weak of mind that you require a belief in God to act rightly."
Notice, however, an important distinction. The Christian is saying that if God does not exist there is no morality, i.e., since there is morality (it is apparent to all) then God exists. He is not saying that his belief in God makes him moral, but that the existence of God defines morality in the world as we experience it.
