Tell me, you who desire to be under the law, do you not listen to the law? For it is written that Abraham had two sons, one by a slave woman and one by a free woman. But the son of the slave was born according to the flesh, while the son of the free woman was born through promise. Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free, and she is our mother. For it is written,
“Rejoice, O barren one who does not bear;
break forth and cry aloud, you who are not in labor!
For the children of the desolate one will be more
than those of the one who has a husband.”
Now you, brothers, like Isaac, are children of promise. But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now. But what does the Scripture say? “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.” So, brothers, we are not children of the slave but of the free woman.
For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery.
~ Galatians 4:21-31; 5:1
Welcome to the Gospel as described in the Old Testament! Paul, still revealing to them the folly of placing themselves under the law again when they have finally been set from it, now explains to them the doctrine of election and what it means to be free from the law. In describing the two children of Abraham, he is able to reveal to them the impossibility of works to fulfill the promise of God and show how God’s plan of redemption brings complete freedom from the law.
Briefly, Abraham was promised that he would father a great nation. This promise was fulfilled both in physical children (the nation of Israel) and in spiritual children (obedient Jews and later Christians). The problem was that Abraham was impatient and, when his wife determined that she was now no longer able to bear children she asked that her husband (who was 86) would, uh, procreate with her young handmaiden. He reluctantly obliged and his wife’s handmaiden, named Hagar, bore him a son named Ishmael. See what happened here? They quit waiting on the Lord and instead acted on their own. So, Abraham finally created a child! This should be the child of promise, right? Nope! God tells him that Ishmael is not the child of promise, and that he will not take over the blessing (and carry the covenant) of Abraham. God does, however, bless Ishmael and he becomes the father of the Arab people who are still a mighty nation to this day.
About 13 years later, God visited with Abraham and reminded him of his promise to make him the father of a nation. His wife, now much older than before, laughed at the notion. God, on hearing he laugh, says that they must name their son “Isaac” which means “laughter”. 1 year later, God fulfills that promise and Sarah bears him a son.
See the difference here? Isaac is the child of PROMISE whereas Ishmael is the child of WORKS. Abraham’s wife believed that God would not make good on His promise to Abraham and therefore, by her own effort, sought to do the work that God had said that He would do. He does not, therefore, honor that act, but acts on His own at a time of His choosing to show that He is the one with the power over that situation. Carrying this further, Ishmael is the son of a slave and as a result is under slavery himself, whereas Isaac was born free as a child of the father. We, as Christians, are dead to the law and buried at our conversion (symbolized in the act of baptism) and we are born again as a child of the Spirit, free from our previous obligation to the law.
Given that information, Paul proclaims that, Jesus has set us free fro the yoke of slavery to the old ceremonial laws. We, as a new creation in Christ, are no longer bound to the law like Ishmael was, but we are free in Christ, like Isaac.