All men are in trouble with God due to a broken law which was revealed prior to the New Testament. This very day, those outside of Christ are condemned due to transgressions of a law revealed prior to the New Testament. When Jesus became a curse for elect Jew and Gentile in the first century, he did so based on law revealed prior to the New Testament. Trace this law (and its curse) back to its revelational origins and you end up in the Garden of Eden, not Sinai (that does not go back far enough). Sinai, in a sense, is a recapitulation of the Garden. That’s why our Confession says, “The same law that was first written in the heart of man continued to be a perfect rule of righteousness after the fall, and was delivered by God upon Mount Sinai, in ten commandments…” (19:2; cf. 19:5, “…the moral law binds all men…”). When Paul deals with justification in Romans 5, he goes back to Adam as the disobedient federal head of the old race and Christ as the obedient federal head of the new race. Christ was obedient to the law (moral law, law written on heart via the creative/revelational finger of God) as revealed prior to his incarnation, first in the Garden of Eden but broken by Adam, then republished on stone tablets under the Old Covenant. The promise of the renovation of all souls in the New Covenant includes the promise of the same law written on all the hearts of all New Covenant members. Jeremiah 31:33 seems clear to me that the law to be written on the heart is the same law written previously on stone tablets. The commands of the New Testament, such as the one anothers, are positive laws (i.e., laws added to the moral law) suited to regulate the new covenant community. The Old Covenant also had moral law and positive law.
Denying moral law as a constant, non-dynamic principle ends up tinkering with the grounds of our justification. The ground of our justification is Christ’s obedience to the law all men have broken, which existed prior to the publication of the New Testament and prior to the promulgation of the Ten Commandments on Sinai.