Now that doesn’t mean that God is irrelevant to ethics; far from it. For one thing, only part of the natural law can be known without reference to God. For example, that murder, lying, adultery, dishonoring parents, etc. are contrary to the good for us can be known from an examination of human nature alone. But the fact that God exists naturally has moral implications of its own, and since for A-T the existence of God can also be known through natural reason, there are certain very general religious obligations (such as the obligation to love God) that can be known through reason alone, and thus form part of the natural law. (Indeed, these are our highest obligations under natural law.) Then there is the fact that the natures of things, including human nature, derive ultimately from those ideas in the divine intellect which form the archetypes by reference to which God creates. (In this way morality is for A-T neither independent of God nor grounded in arbitrary divine commands, as I explained in a post on the Euthyphro objection.) Furthermore, for A-T, a complete account of moral obligation requires reference to God as legislator (even if moral obligation can proximately be explained by reference to the natural end of the will). Finally, divine revelation is also needed for a complete account of everyday moral life. For one thing, divine revelation discloses certain details about morality that the human intellect is too feeble reliably to discover on its own. For another, some aspects of the natural law are so demanding that many people are capable realistically of living up to them only given the hope of a reward in the hereafter, of the sort divine revelation promises. (Again, all of these issues are discussed in Aquinas. See chapter 8 of the first volume of Michael Cronin’s The Science of Ethics for a useful treatment of the proximate and ultimate grounds of moral obligation.)
All the same, since to a large extent the grounds and content of morality can be known from a study of human nature alone, it follows that to a large extent morality would be what it is even if human beings existed and God did not. For, again, morality is not based in arbitrary divine commands any more than scientific laws are expressions of some arbitrary divine whim. From the A-T point of view, “divine command theory” (or at least the crude version of divine command theory that takes the grounds and content of morality to rest on sheer divine fiat) is, I would say, comparable to occasionalism, and similarly objectionable. (Cf. my recent post on Ockham.)
Per impossibile... after all we cannot come into existence on our own. The goods of human nature would stay the same, though something would be missing, our ultimate end.