We have noted earlier that the commandments of Grace are tougher and more demanding than the commandments of the Law. The Bible tells us that the Law was given through Moses, but grace and truth came with Jesus Christ (John 1:17). Jesus ushered in the era of Grace. Not that grace was absent in the Old Testament; but that grace could only be available even in the Old Testament because of the Lamb that was slain from the foundation of the earth. And, when Christ came Grace came in reality, for until then everything was only shadows (Col.2:17). Therefore, it says, "Grace and truth came with Jesus Christ".
The commandments of Grace, therefore, supersede the commandments of the Law. Grace teaches us true righteousness (Tit.2:11,12; Matt.5:20).
Thus, certain things that were allowed in the Old Testament (like divorce, swearing, polygamy, and tit-for-tat ethics) are not allowed anymore in the New Testament (Matt.5:31,34, 38,39). Most of these things were allowed because of the hardness of human hearts, but God never originally intended them so (Matt.19:8). However, in the Age of Grace when His Grace transforms our hearts, we are called to love our neighbor as ourselves and to pray for our enemies, we are called not to resist evil people but turn our left cheek to someone who slaps on our right (i.e. severely insults and humiliates us). The demands of Grace are higher than the demands of the Law.
The commandments of Grace spring from the spirit of the Law not the letter of the Law. Thus, while the OT command only said, "Do not murder" and "Do not commit adultery", the command of Grace tells us to not even get angry with our brother without cause and tells us looking at a woman lustfully is equal to committing adultery (Matt.5:22,27).
The essence of the command of Grace is Love (Rom.13:8,10; Gal.5:14; James 2:8).
The good news is that this Love is poured into our hearts through the Spirit in the New Testament; therefore, the commandments of God are no longer burdensome or impossible (Rom.5:5; 2Cor.12:9; 1 John 5:3).