Galatians 3:15-20

Brothers and sisters, I speak in terms of human relations: even though a last will and testament is just a human covenant, yet when it has been signed and made legally binding, no one sets it aside or adds to it [modifying it in some way]. Now the promises [in the covenants] were decreed to Abraham and to his seed. God does not say, “And to seeds (descendants, heirs),” as if [referring] to many [persons], but as to one, “And to your Seed,” who is [none other than] Christ. [Gen 13:15; 17:8] This is what I mean: the Law, which came into existence four hundred and thirty years later [after the covenant concerning the coming Messiah], does not and cannot invalidate the covenant previously established by God, so as to abolish the promise. [Ex 12:40] For if the inheritance [of what was promised] is based on [observing] the Law [as these false teachers claim], it is no longer based on a promise; however, God granted it to Abraham [as a gift] by virtue of His promise. Why, then, the Law [what was its purpose]? It was added [after the promise to Abraham, to reveal to people their guilt] because of transgressions [that is, to make people conscious of the sinfulness of sin], and [the Law] was ordained through angels and delivered to Israel by the hand of a mediator [Moses, the mediator between God and Israel, to be in effect] until the Seed would come to whom the promise had been made. Now the mediator or go-between [in a transaction] is not [needed] for just one party; whereas God is only one [and was the only One giving the promise to Abraham, but the Law was a contract between two, God and Israel; its validity depended on both].