If there were no such thing as green, what color would green things be? MU.
Kierkegaard talked about all this in Fear and Trembling a long time ago. Was Abraham sacrificing Isaac immoral? If you call morality the universal of that time, then you have to suppose it was. But in the story we know that God told Abraham to do it.
Anyone who defies the universal in favor of their own experience of God, the universal has no way to judge. No argument on the basis of the universal morality can call Kierkegaard's knight of faith back from the quest.
"And if an external objective morality does say that the universe should occupy some horrifying state... let's not even ask what you're going to do about that. No, instead I ask: What would you have wished for the external objective morality to be instead? What's the best news you could have gotten, reading that stone tablet?
"Go ahead. Indulge your fantasy. Would you want the stone tablet to say people should die of old age, or that people should live as long as they wanted? If you could write the stone tablet yourself, what would it say?
"Maybe you should just do that?
"I mean... if an external objective morality tells you to kill people, why should you even listen?"
This is a restatement of ethical egoism. It assumes that what you want is right, so you should just do what you want. If an external objective morality tells you to save people's lives, why should you even listen? Morality is not the only target in the sights of this argument; amorality takes it just as badly in exactly the opposite fashion. (That doesn't mean that you have to pick something besides morality and amorality, it just means the argument is wrong.)
If your desires are messed up (any pedophiles reading the blog?), there is no prospect of change. No one can persuade you out of your egoist position, no argument works. But that's evidence that it's the wrong position to be in, unfalsifiable and unjustifiable.
If you believed this, you might as well say, I am perfect. The world can teach me nothing. I can receive input but never change. It turns out you're incorrect, that's all.