21 Tell me, you who want to be under the law, are you not aware of what the law says? 22 For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 23 His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise.Yes we are back at Galatians after (yet another) break and we'll take up one of the more fascinating passages in Galatians as Paul allegorically interprets Genesis 21 and its surrounding context. While allegory is looked down upon now, that has not always been the case. Second temple Judaism and Christianity at least up to the Reformation embraced allegory, as Paul clearly does here. Far from being a throw away or small addition to his argument, Paul uses the allegory to drive one of his key exhortations to the Galatians, to have them kick out the false teachers. Some interpreters have claimed that Paul isn't actually resorting to allegorical interpretation here but instead is interpreting typologically. I think that opinion results more from their own uncomfortableness with non-literal methods of interpretation. While Paul isn't going wild in his allegory, it still clearly is allegory.
24 These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. 25 Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children. 26 But the Jerusalem that is above is free, and she is our mother. 27 For it is written:
“Be glad, barren woman,
you who never bore a child;
shout for joy and cry aloud,
you who were never in labor;
because more are the children of the desolate woman
than of her who has a husband.”
28 Now you, brothers and sisters, like Isaac, are children of promise. 29 At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now. 30 But what does Scripture say? “Get rid of the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.” 31 Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman. (NIV)
One of the keys to interpreting this passage is seeing who's who. Paul here contrasts two groups. On the one side is Paul and those who promoted a law-free Gentile mission. On the other side are the Teachers and those who required Gentiles to be law observant. The Teachers are the children of Hagar, the ones who are enslaved and thus continue to beget children in slavery, children who oppress the children born of the promise.
Paul also rips a portion of the Jerusalem church here as well. The Teachers claimed their superiority to Paul because they were backed by part of the Jerusalem church. Paul had no official backing. However, Paul claims backing from a better Jerusalem, the heavenly Jerusalem. Any who require Torah observance to be part of the people of God are going back to the old order before Christ, back into slavery (similar to his argument in 4:8-11). Patrilineage guarantees nothing. Only children of the promise (who's identity was revealed earlier) are free.
The citation in the middle of the passage comes from Isaiah 54:1. To understand Paul's usage here, first we need to notice the wider context in Isaiah. It directly follows Isaiah 53 and the description of the suffering servant. Isaiah 54 is the beginning of a song of praise for the eschatological victory that God's people will experience. By citing the verse at the hinge (remember there were no chapter divisions in Paul's day) Paul here is declaring that it is through union with God's people through Christ's work on the cross that the Gentiles have hope. They know that they will some day enter glory, and it's not because of the Torah, but because of the work of Christ alone. The choice of passage is also brilliant by Paul for the echoes back to the Genesis passage at hand (as well as to the story of Hannah).
As mentioned before, Paul concludes with the command to kick out the Teachers from the congregation. There is no room in the body of Christ for those who seek to marginalize and enslave God's children with any kind of law.