This “commandment” is to love one’s brother, not the Mosaic Law. Unlike the Mosaic Law, which God established on Mount Sinai, the commandment to love one’s brother is eternal and predates Sinai. Thus when Cain killed Abel he sinned and broke this commandment even though “thou shalt not kill” had yet to be engraved on the stone tablets at Sinai.
***
That much we can agree on! More or less. Large swaths of the Mosiac law were for a specific time and place and specifically for the purpose of foreshadowing Jesus, in particular the systems of circumcision, cleanliness and uncleanliness,, and Temple sacrifices.
I would argue that the Ten Commandments are part of Mosaic Law, but also part of natural law—the kind that Paul says is written on the heart, or at least that’s what I mean when I write it. YMMV on definitions.
Whatever the case, it’s very important to teach and live the commandment of love. I may not believe that I can earn salvation from it, but love is the natural result of salvation—and it’s something we should practice to keep the ‘old man’ in check until Jesus returns anyways.
.
.
>> “Thus when Cain killed Abel he sinned and broke this commandment even though “thou shalt not kill” had yet to be engraved on the stone tablets at Sinai.” <<
It is obvious to even the most cursory reader of Genesis that all the commandments had to have been related to Adam.
How did Abel know a flesh and blood sacrifice was necessary?
How did Noah know which animals were clean or unclean?
.