There is a great post on As Bereans Did about the common arguments legalists use to "prove" that the law is eternal (whihc it is not).
One of those so-called "proofs" is the moral law. It is an excellent article, with many well thought out points, but I picked this part of it to highlight
MORAL LAW
Maybe by this point you are thinking, maybe the national and ceremonial laws aren't eternal but the moral law has to be. Supposedly the moral law flows naturally from God's own moral nature, therefore the moral law is eternal because God's moral nature is unchanging. Then why not say that? Why not claim "the moral law" instead of "the law"?
I'll tell you why. People do not make this argument to get others to stop murdering or coveting. What they want is to justify the non-moral laws on their cherry-picked list, like tithing, meats laws, holy days, and the weekly Sabbath.
Let's ask that tough question, though. Is the moral law eternal?
What about the law against adultery?
That's a law everyone can agree is a moral law. How could that exist before there was marriage? In the future, no one will marry (MAT. 22: 30). The law about adultery does not exist if marriage does not exist. Just like the Sabbath without days.
The moral law prohibiting adultery is not eternal.
What about the law against murder?
How can the law against murder exist before humans could die, or continue on after all humans are immortal? All humanity will eventually be immortal. The law about murder does not exist if morality does not exist.
The moral law prohibiting murder is not eternal.
What about the law against covetousness?
How can the law against covetousness exist after the fullness of the Kingdom has come, and we have fully received the inheritance we are promised in Jesus, and we are fully possessors of all things? How do we covet what is already ours? In the future there will be no such thing as limited resources. Everyone will have more than plenty, and then some. The law about covetousness does not exist if limited resources does not exist.
The moral law prohibiting covetousness is not eternal.
What about the law against idolatry?
How can the law against idolatry exist after everyone lives in the direct presence of the true and living God? Who among us, when we live in the fullness of the Kingdom of God, would ever, ever turn back to worshiping anything less? It's absurd! The law against idolatry does not exist if there is worship of other gods/things/etc does not exist.
The moral law prohibiting idolatry is not eternal. This one has the best chance of being eternal, but it seems somewhat childish to me to presume perfected beings will need a law.
"But those acts are still wrong even if they are impossible to commit," someone is no doubt saying right now. That's like saying it's a sin to kill a dinosaur. They're are no dinosaurs, but it's still a sin to kill one. Makes sense? No. And here we go, back to myriads of unknown laws governing things that do not and might never exist.
"The law is eternal..." STOP! No, it isn't. Not even the moral laws are eternal.
Turns out eternality is not an attribute of moral law and never mattered in the first place. This entire argument is a pointless exercise in futility, and a distraction.
This is a problem some people solve by leaving it obscure and refusing to deal with it. Somewhat reminds me of the situation in my last post, "Willful Ignorance". It is easier to bury the head or to make sweeping generalities than to investigate it and realize you've invested so much of your time, energy, and money in a mistake. Common Legalist Arguments - Part VI