Edward Feser has a thorough response to Stephen Law’s “evil-god” challenge. Law’s challenge is that no theist system has any more reason to posit a (morally) good God than to posit a (morally) evil one. Feser observes that Law has very little awareness of what classical theism called “evil”, and that his arguments against what he does understand are ineffectual. I’m not interested in Law’s challenge since it is a dialectical game that can continue for as long as he refuses to define what he means by good or evil, even in a moral sense. So long as Law is just putting one set of evidences with their respective hypotheses, then he’ll have a case for God being good and evil. This is a feature of dialectics, however, not a feature of theology. I’m interested in another point.
I’ve ranted more than once about the difficulties in calling God “morally” good (or morally evil, for that matter) since it’s hard to see how anyone could argue that the divine nature is eternally chaste, moderate with his food, thrifty with his income, and so forth. If there is any sense to God being morally good, it is that his will is upright, which is to say that it is perfectly ordered to the goals that it ought to pursue. But on this account classical theism throws up another impediment to Law’s evil-god challenge, for there is a very long tradition that argues that God just is the goal that a will ought to pursue. There is a very long tradition holding that the ultimate rectitude of a will consists in striving to become as much like God as possible, and thus that the attainment of the divine life is the goal of all moral action. On this account, God is a moral being simply because he exists – indeed, has possesses the goal of morality infallibly simply by being divine.
Arguments for this sense of happiness are clear in the pre-Socratics, they are given their locus classicus in Plato’s Theatetus and Book X of the Ethics, and they are found all over the place in Scholastic thought. St. Thomas gives any number of rational arguments for the claim here and here (and in the many chapters around this latter point).