This will seem a harsh judgment on the many theists who subscribe to what is called Divine Command Theory — the view that what is morally good is constituted by what God commands. Defenders of D.C.T. will say that their theory explains a variety of things about morality that non-theistic accounts of moral value cannot, and that it should be preferred for that reason. For example, they will say that atheists cannot explain the objectivity of morality — how there could be moral truths that are independent of any human being’s attitudes, will or knowledge, and how moral truths could hold universally.
It is true that D.C.T. would explain these things. If God exists, then He exists independently of human beings and their attitudes, and so His commands do, too. If we didn’t invent God, then we didn’t invent His commands, and hence didn’t invent morality. We can be ignorant of God’s will, and hence mistaken about what is morally good. Because God is omnipresent, His commands apply to all people at all times and in all places.
That’s all fine. It would follow from D.C.T. that moral facts are objective. The problem is that it wouldn’t follow that they are moral. Commands issued by a tyrant would have all the same features. For D.C.T. to explain morality, it must also explain what makes God good.
The problem I’m pointing to is an ancient one, discussed by Plato. In his dialogue “Euthyphro,” the eponymous character tries to explain his conception of piety to Socrates: “the pious acts,” Euthyphro says, are those which are loved by the gods.” But Socrates finds this definition ambiguous, and asks Euthyphro: “are the pious acts pious because they are loved by the gods, or are the pious acts loved by the gods because they are pious?”