Plato's dialogue Euthyphro presents a logical problem about the meaning of piety (or more broadly, the Good) in a polythestic society. The basic question: "Is something good because the gods approve it, or do they approve it because it is good?"
In its original polytheistic context, it was a way for Plato to point to the fact that there is some external moral standard that even the gods must bow to.
Occasionally, modern skeptics or atheists will attempt to use the same line of argument to make the idea of a monotheistic God problematic. This modern adaptation follows the these lines.
"Is something good because God approves it, or does God approve it because it is good?"
1. If we answer that something is good because God approves it, then it would seem that the good is merely arbitrary, and that tomorrow God could declare cannibalism and adultery good. Clearly a God of this sort wouldn't be a God worthy of worship.
2. If we answer that God approves something because it is good, then it would seem that there is something superior to God that even God must obey. This sort of God clearly isn't omnipotent, and thus isn't the sort of God that monotheistic religions worship.
But the problem with this line of argument is that it presupposes a contrast between God and the Good which isn't possible in the classic monotheism of Augustine, Aquinas, etc. In their conception, there can be no contrast between God and the Good because God is the Good. The self-identity of God as the Good renders the question itself nonsensical.
"Is something good because the Good approves it, or does the Good approve it because it is good?"
The question no longer poses a problem. The answer is simply, yes, it is good because the Good approves it, and yes, the Good approves it because it is good.
This classic conception of God as the Good is shared across the monotheistic religions, but it was originally worked out specifically in relation to the idea that Christ is the Logos. The word Logos has many meanings in the philosophical tradition, but they include things like Reason, Moral Law, Wisdom, etc.
Because of this, arguments which depends on a contrast between God and the Good can only succeed with a sort of super-voluntaristic understanding of God's sovereignty which override an understanding of God's unchangeable nature as the Good itself.