Among beings there are some more and some less good, true, noble and the like. But “more” and “less” are predicated of different things, according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest and, consequently, something which is uttermost being; for those things that are greatest in truth are greatest in being, as it is written in Metaph. ii.So there is a best, a maximally good, thing, that is the measure of all things as far as goodness goes. Aquinas believes that this maximum is also the cause ("as fire [...] is the cause of all hot things," i.e. not the cause in the ordinary, contemporary sense of the word), and in the case of perfections such as goodness this maximum and cause is God.
But if God is the standard of goodness, does it make sense to call him good? People often say it does not, but I think it could. In another context Wittgenstein says this:
There is one thing of which one can say neither that it is one metre long, nor that it is not one metre long, and that is the standard metre in Paris.--But this is, of course, not to ascribe any extraordinary property to it, but only to mark its peculiar role in the language-game of measuring with a metre-rule.One cannot say that the standard is or is not one metre long because... Actually, because nothing. Wittgenstein does not say that one cannot do so because of the peculiar role the standard meter plays in the language game. He says that saying that one cannot say that it is or is not one meter long is to mark that role. He does not say that one must mark this role. I can dub an arbitrary length "one metre" or, since that name has been taken, "one meeetre," say, but dubbing and describing are different acts. So of the thing that is now the standard metre, before it was the standard metre, we could say "This is one metre long." But this is an act of dubbing, not of description. Saying it is defining "one metre," not providing information about the stick in question. And once "one metre" has been defined that way, it provides no information to say that the standard metre is one metre long.
But providing information is not the only reason to speak. So if God is the standard of goodness then it will not tell anybody anything to say that God is good, but there might be other reasons to say it. It might be a reaffirmation of one's commitment to the language-game of judging goodness relative to God. It might be something else. It isn't necessarily nonsense just because it isn't informing someone of a fact.
What if something changes? Say we see the standard metre visibly grow or shrink. There is no knowing what we do, it seems to me. It would be weird. We might ignore it, or feel the need to throw away all our old rulers and tape-measures, or we might get a new standard metre. Similarly if God appeared to change, say by ordering you to kill a child or by threatening to destroy an entire city or otherwise behaving in what strikes us a surprising or ungodlike way. We might ignore this odd behavior, pretend not to have noticed it. Or we might insist that this could not be God, and so reject the appearance as illusion. Or we might accept that whatever God does or commands must be right, however wrong it might seem to us. Or we could start defining 'good' in some other way.
There isn't one thing that we would have necessarily to do, but it's also not as if we simply choose how we react to surprises. Sometimes we find ourselves reacting a certain way and there's nothing we can do about it. So our standards are neither dictated by the universe nor chosen at will by us. A bit like faith in God. (I'm not sure that I have much of a point to make, if you're wondering about that. Just thinking out loud really.)