What are the origins of the theory, present in the "Didascalia Apostolorum", about the existence of a "First Law" and a "Second Law"? I recently read a study on the "Didascalia Apostulorum",probably composed in Syria in the first half of the third century AD.

I was very impressed by the theory contained therein, according to which it is necessary to discern, within the commandments of the Pentateuch, between a "First Law", also simply called "Law", and a "Second Law". The precepts of the "First Law" are represented by the Decalogue and more generally by the rules of the Torah that the Creator issued before the Israelites committed idolatry by worshiping the golden calf.
The commandments of the "First Law" are eternal and constitute a "sweet yoke" for the human being; after the adoration of the golden calf, the Creator instead issued a "Second Law", made up of a "heavy burden" of rules which take on a substantially punitive and afflictive guise. In this context, Jesus Christ would come to repeal the "Second Law" and thus free the faithful from this burden.

Do you know the historical-theological origin of this theory?
I think the theory is based on a faulty understanding of the Scriptures.  


Jesus, when asked about the greatest commandment in the law, answered:  

"...'You shall love the Lord your God with all your heart, and with all your soul, and with all your mind.'  This is the first and great commandment."  [Matthew 22:37, 38, MEV].  
And Jesus went further:  

"And the second is like it: 'You shall love your neighbor as yourself.'  On these two commandments hang all the Law and the Prophets." [Matthew 22:39, 40]  


It should be noted that neither of these commandments is included in the Ten Commandments, and that both were given after the golden calf idolatry of the Israelites:  therefore, both of these would fall under the theory's classification of "Second Law."  

Two commandments of the "'heavy burden' of rules" were judged by Jesus to be the two greatest commandments of the entire Law.