Paul continues to say that God made a promise to Abraham and that it is was made to his offspring. That promise, as I have been teaching my kids in Children's Church, was the Rescuer (Jesus) who would take away our sins.
Think about that for a second. If a man-made contract cannot be changed, how much harder is it to change a contract if God is the one making the promise? There is nothing that can be done to change the promise of salvation and those of us who are firm in that, through our faith in Christ, have nothing to fear.
Galatians 3:26 - For in Christ Jesus you are all sons of God, through faith.