In Galatians 4:21 Paul returns again to Scripture in order to conclude his argument about relationships. He asks those, who wish to embrace Judaism, if they really took seriously the claim the Law has upon them. Perhaps it is true that often we accept something that appears to be true, even Biblical, but we never really consider the consequences the truth under consideration has upon the truth as a whole. Certainly much of Judaism is based upon the word of God, yet Jesus called at least some of what Judaism taught “your tradition”, that is, tradition of men and said that it contradicted or took away the power of the word of God (Matthew 7:8, 13).
Paul draws upon an allegory in Galatians 4:22-31 that concerns Ishmael and Isaac and Hagar and Sarah. His point in doing so is that the allegory represents two covenants (Galatians 4:23-24), one of the flesh and one of promise. The flesh, Hagar, represents bondage to the Law, while the promise, Sarah, represents faith and freedom in the authority of Christ. What is interesting is that, although Hagar was the bondwoman in Genesis, the natural descendants of Abraham through Sarah were in bondage according to Paul (Galatians 4:24-25). Those who are free are the spiritual descendants of Abraham, both believing natural descendants of Abraham and believing gentiles, who have no natural connection to him (Galatians 4:26-28). Rather, the gentile connection to Abraham is spiritual through his promised Seed, Christ, in whom all nations would be blessed (Genesis 18:18; 22:18).
In Genesis 21:9 it was Ishmael, the son of Hagar, who persecuted Isaac, the child of promise. Nevertheless, according to Galatians 4:29, it was the Jews, the natural descendants of Abraham who persecuted the children of promise (believers, whether Jewish or gentile).
Notice that in Galatians 4:27 Paul claims the children of the barren one are many more than the wife who has a husband. What Paul is doing here is pointing to another allegory which is offered by Jeremiah in Lamentations 1:1-8 where he identifies Jerusalem or Zion as a widow. The text says the city is like a widow and describes her desolate state. Her walls are destroyed, and her gates are sunk into the ground, her king and princes are in captivity and she is bereft of her sons (cf. Lamentations 1:16). Jerusalem mourns and enters her widowhood without hope of redemption.
Nevertheless, the God of Israel is merciful and loving. He remembers his promises and is willing to lift Israel out of her trouble. Notice in Isaiah 54:1-6 Jerusalem is described as the barren one, having no children (princes or heirs to the throne). The Lord says he, himself, will build up her walls and place her gates in their proper places (Isaiah 54:11-12). How is this done? The Lord describes himself not only as her Husband (Isaiah 54:5), but also her Redeemer. The context of the title, Redeemer, is one of giving seed to the barren one just as it is used of Boaz in Ruth 4:14. The words: “For thou shall break forth on the right hand and on the left; and thy seed shall inherit the gentiles…” points to the Gospel going out and bearing fruit among the gentiles, bringing them in as sons of Abraham—sons according to the promise, or of faith.
Contrasting this promise with the Galatians embracing Judaism Paul shows them what the Law says: “Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman” (Galatians 4:30). Hagar was cast out in Genesis 21, and shortly after the prophecy of Isaiah 54, Judah would be cast out of the Promised Land. Paul’s point was that, shortly after his letter to the Galatians, the Jews would be cast out of their land once more after the Jewish war with Rome in 66-70 AD, showing how much in common they had with Hagar.
On the other hand, the sons or children of the freewoman would be heirs of the promises (Galatians 4: 31). Therefore, as children of the freewoman, we need to stand in the liberty in which Christ has made us free (Galatians 5:1). That is, we need to be careful about what we submit ourselves to obey. Christ wants us to be good citizens and obey the laws of the land, because this is supposed to reflect good upon Christ. However, to submit to law whether civil or religious as a means of serving God’s ends is wrong. God never intended to accomplish his goal through law, and to zealously seek to do so puts law in the place where Christ should be.