Galatians 3:15-16

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]