Socrates famously posed the dilemma: “Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?” Later, this dilemma forced itself upon monotheistic religion, in the form of “Are things moral because God commands them, or does God command them because they are good?”
Taking the first horn of the dilemma commits one to a divine command theory of ethics, the latter to something more like a natural law position. Problems appear on both horns for the Christian, because the first approach seems to imply moral absurdities (like saying that child-rape would be good if God commanded it) and the second metaphysical ones (leaving morality as a kind of unexplained surd, existing independently of the putative First Cause).
For this reason Christian thinkers have tried to resolve the dilemma. One common approach is to say that the moral standard is God’s own nature, which he then commands that creatures reflect. I think this answer is partly right, but incomplete. By virtue of divine simplicity, it is true that Goodness is identical with God, since God is the source of good, and must be good, and being identical with his nature, must be goodness itself. Yet this answer is incomplete, for it’s not clear how divine goodness relates to what is good for human beings, which is what the dilemma was originally talking about.
Christians want to affirm that human goodness somehow finds its source in God (again, he is supposed to be the ultimate explanation for all things), but nevertheless want to avoid the problems that seem to come with a crude divine command theory of ethics. How can this be done? It seems to me that a kind of Thomistic metaphysics is the only good answer. One could try to avoid this route by taking a more voluntaristic and nominalistic metaphysics, but such an approach seems to entail the same moral problems that the crude divine command theory does. But then how can one avoid the other problem, of making morality an ontologically ungrounded surd?
The way around this problem comes in recognizing that what is good for human beings is determined by their nature, but that that nature is created by God. So while it is true that morality as such is not essentially a divine positive law backed up by threats of force, nevertheless it is derived ultimately from God as Creator. And further, since God’s nature is the law for his own actions, his creative activity partakes of the goodness of his own nature.
Thus one can avoid both horns of the dilemma and provide an intelligible connection between the goodness of God and morality if one takes the path of Thomistic natural law.