“So you are no longer a slave, but God’s child; and since you are his child, God has made you also an heir” (4:7). Through our identification with God’s Son, who became human for our sakes, we become God’s adopted sons and daughters and receive “the Spirit of his Son” in our hearts. Paul said that if we return to the Law and try to impress God and others (8-11), we are behaving like slaves. But in Christ, we are “heirs” of all his promises (21-31).
“Now you, brothers and sisters, like Isaac, are children of promise” (28). Isaac was a gift, not a reward for hard work. By faith in God’s Son, we receive the Spirit and all the Father’s good gifts.