Divine nature theorists believe that goodness is grounded in a likeness to God's essential nature. As they would say, it is good to be loving and kind and just because God is loving and kind and just. In other words, the property goodness reduces to being a member of the set of properties that God holds necessarily. But one might wonder, why do we need God at all? Wouldn't it be simpler to say that goodness reduces directly to that collection of properties, without referencing God's nature? Why shouldn't we rather say that what it is to be good is just to be loving or to be kind or to be just and so on? This revised theory would seem to have all the same explanatory power as divine nature theory, but would be a great deal more parsimonious for not requiring the existence of God.
I once suggested this to a friend, and he responded by saying that in such a case there would be nothing all good things had in common. But this seems odd: surely for there to be such a thing as goodness, there must be something all good things have in common. Intuitively, he thought, there must be something that brings all these properties together (and in the darkness binds them). At the time I was unaware of the classic counter examples: species being an obvious case of a bundle property, being made up of a collection of properties which do not necessarily all share something in common. There are no necessary and sufficient conditions for being, for example, a lion. By analogy, we might think there need not be necessary and sufficient conditions for being good.
On top of this I now think there are two even stronger objections to this line of reasoning:
First of all, divine nature theory itself suffers from the same problem my friend had with treating goodness as a bundle property. Why is this? Because being like God is a bundle property. There is nothing all of God's essential attributes have in common, other than being those attributes God necessarily instantiates. The only difference is that we've given this collection a name ("God's essential nature") and ground goodness in a likeness to it. Theists may suggest that on greatest being theology, the divine nature theorist could think that all God's essential attributes have in common their being great-making. But greatness is no less metaphysically queer and demanding of explanation than goodness, so this move would only push the need for grounding a step back without really explaining anything substantial.
Secondly, there is something all good things have in common without God after all: their normativity. If something is good, then there is reason to conduct oneself towards it certain ways. There is reason to desire good things, to perform good actions, and to praise the good behavior of others. Furthermore, most would say this follows from our very concept of goodness itself. We therefore need not talk about God's essential nature to explain what it is to be good.