“For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise” (v18). Paul showed clearly that the Jews had never been saved because of law-keeping; their relationship with God was always based on grace: His loving initiative and promise. The law, given much later, certainly had good purposes, but it was never life-giving (v17-24). Paul said that having become God’s children through faith in Christ, we can’t go back to trying earn God’s approval through “the works of the law,” which never saved anyone anyway (v2, 26).
Faith in Christ makes us “Abraham’s offspring,” God’s adopted sons and daughters and heirs of all His wonderful promises (v29).