Paul now wraps up his point of justification before God coming by faith alone in vv.21-31 and gives a Biblical example for the believers in Galatia.
Tell me, you who want to be under the law, do you not understand the law? For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. But one, the son by the slave woman, was born by natural descent, while the other, the son by the free woman, was born through the promise. These things may be treated as an allegory, for these women represent two covenants. One is from Mount Sinai bearing children for slavery; this is Hagar. Now Hagar represents Mount Sinai in Arabia and 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 woman who does not bear children;
break forth and shout, you who have no birth pains,
because the children of the desolate woman are more numerous
than those of the woman who has a husband.”
But you, brothers and sisters, are children of the promise like Isaac. But just as at that time the one born by natural descent persecuted the one born according to the Spirit, so it is now. But what does the scripture say? “Throw out the slave woman and her son, for the son of the slave woman will not share the inheritance with the son” of the free woman. Therefore, brothers and sisters, we are not children of the slave woman but of the free woman.
Paul gives an example of justification by faith using Sarah and Hagar. In the ancient world, the status of the mother had a large influence on the status of the son (vv.21-27). Paul is saying, simply, that one son is born under law, to the flesh, of the slave, whereas one was born of the free woman, through promise.
He says that this can be viewed as representing a bigger truth: the two covenants between God and man. On the one hand is law, justification by works, living in the power of the flesh. On the other is living by the promises of God, being justified by faith alone, and living in the power of the Spirit. He then turns this to the Galatians,
But you, brothers and sisters, are children of the promise like Isaac.
Having made the big-picture point with a Biblical example, Paul now gets personal again. Urging the Galatians to remember that they are children of the promise like Isaac, he shows them that, look, as at that time the one born by natural descent persecuted the one born according to the Spirit, so too are these law-bringing Judaizers making your life difficult. As these false teachers with their false doctrine are here represented by the slave woman and her son, Paul is saying pretty clearly that they ought to be thrown out. The churches in Galatia had no shared inheritance with false teachers and Paul’s position is pretty clear: law and grace are fundamentally incompatible and you, churches in Galatia, are under grace (v.31, cf. Romans 8.17).
For you and for me, this is not about cutting ties with anyone of a different worldview. Yes, talk to people who see the world differently from you. Your Christian, Jesus-focused lens on life is robust enough to withstand conversations and friendships with others who do not share it. In context, Paul is counselling the Galatians to remove false teachers from their midst, not to disavow anyone who thinks differently to them. When we reach a point in our lives when we have truly accepted the truth of God, of His Son, and we are living by the power of His Spirit, we are able to spot false teaching as false teaching, as Paul is doing here. Then the advice and counsel is much stronger: spend no time with nor pay any attention to them (cf. principle of 2 John 1.9-10). Yes, talk to people and spend time with people who see the world differently to you. But, never forget that you, brothers and sisters, are children of the promise.