“In Christ Jesus you are all sons of God, through faith,” writes Paul to the church at Galatia. When you are baptized into Christ, you “put on Christ.” (Galatians 3:26, 27) And in belonging to Christ, “you are Abraham’s offspring, heirs according to promise.” (Galatians 3:29)
Now Paul is telling the Galatians that as long as the heir is still a child, he “is no different from a slave.” (Galatians 4:1) This heir, as a child, remains under the guardianship of the “elementary principles of the world” and under the management of the law. (Galatians 4:3,4) But God, at the “fullness of time,” sends His only begotten Son, Jesus Christ into the world “born of woman, born under the law, to redeem” you who are still children. At this point in time, you become heirs, “the owner[s] of everything” God has prepared for you. (Galatians 4:4,5,2)
You become sons of God by adoption. And with this adoption, you become brothers of Jesus Christ and also receive the promised Holy Spirit, who cries in your heart, ‘Abba! Father!’ (Galatians 4:6) “You are no longer a slave, but a son, and if a son, then an heir through God.” (Galatians 4:7)
An heir has rights to everything the Father owns. Paul exhorts that there is no reason “to be under the law” for Abraham has “two sons, one by a slave woman and one by a free woman.” (Galatians 4:21,22) You are born of the free woman, so why would you desire to be in the company of the slave woman? What have you in common with the slaves — with those who remain under the management of the law and enslaved to the elementary principles of the world? You, rather, “are children of promise.” (Galatians 4:28)
“Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman.” (Galatians 4:30)