Debates about morality get confusing because the word "good" is poorly defined.
This post is an attempt to make a more refined definition.
First Person Subjective Good (1SG): An action I think I should take given what I know.
First Person Omniscient Good (1OG): An action I think I should take if I had all knowledge.
[I leave open the definition of omniscient for the moment.]
Third Person Subjective Good (3SG): An action I think I should take if I were in a third person's position with their knowledge.
Third Person Omniscient Good (3OG): An action I think I should take if I were in a third person's position with all knowledge.
Evils are actions that are defined the same way but with the words "should not take" replacing the words "should take." I'm leaving open for the moment the question of whether there might be some actions that are free of moral obligations.
The above is not very controversial for the typical human, though humans might disagree on what specific actions are good and what are evil. Generally, 3SG's and 1SG's don't conform, e.g., what I would do in another's position is different from what they actually do. Let's call this subjective moral discord.
One thing that's a bit more controversial will be the definition of a subjectively good person. I'll venture that my knowing what my good is doesn't automatically make me good. I might know what I ought to do, but do something else anyway. In those cases, I would be acting like a subjectively bad person. This isn't an obvious conception. I'll justify it for now by saying that we are often aware that we should do one thing, but do something else on a whim.
Now, we introduce God:
God's 1SG: Actions God thinks he should take.
God's 1OG: Same as God's 1SG.
God's 3SG: Actions God thinks I should take given what I know.
God's 3OG: Actions God thinks I should take.
The hard questions have to do with omniscience.
Do all omniscient beings have the same morality? Put another way, is God's 3OG exactly the same as my 1OG? If I knew what God knew, would our goods coincide?
If omniscient goods do not coincide, then there is no objective good. If two omniscient beings have differing views of the right action, there can be no absolute right or wrong. We'll call this omniscient moral discord. God has the power to punish everyone for not following his orders, and he has the power to compel us to do what he says. However, that doesn't make his ideal actions the definitive good if, say, knowing what he knew, we would have thought it better to create a universe without suffering and without Hell. In such a scenario, an act that God considered good would be evil in my eyes.
The theist can argue that we are not omniscient, so we are in no position to judge whether there is omniscient moral discord. However, this also argues against confidence in objective good arising out of coincident omniscient goods. If there is subjective moral discord, and we cannot imagine what it is to have all of the information, then we really have no reason to believe that omniscient goods are indistinct.