Galatians 3:15-18

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.