This is the final stage in Paul’s scriptural argument against the agitators in the Galatian churches. He has made the point that Abraham was justified before circumcision, rather than after. In fact, Abraham was right with God before the Law was given at all.
Second, the allegory itself seems strange to the modern reader since it is not a modern allegory at all (Aslan is Jesus, Pilgrim’s Progress, etc.). But that is not at all the sort of allegory Paul wants to find in the Sarah and Hagar story. Paul is creating a contrast between the two sons of Abraham, one who was born free, and the other who was born in slavery. The story in Genesis is not an allegory at all, Paul is drawing an allegorical contrast from it. As Ben Witherington points out, Paul is using allegory to contemporize a text, not find “deeper meanings” (Galatians, 330). In other words, this is more like an application, or an analogy in contemporary rhetoric rather than a full allegorization of the original story.
With those clarifications, what is the point of the allegory? These two covenants are contrasted as between the earthly Jerusalem and heavenly Jerusalem. This is obscure, but Paul’s point is to connect the old covenant (the Law) with Sinai, a location outside of the land, in Hagar’s territory, with the new covenant enacted in the “real” Jerusalem in Heaven.
The fact that Paul considers Jerusalem to be under the yoke of slavery is significant. He could be referring to the fact that it is still under Roman rule (the exile continues), but likely as not he is dismissing the earthly Jerusalem because the agitators make a great deal about their connection to the Jerusalem church.
What is surprising is that Hagar represents those who are enslaved by the Law, or Second Temple Period Judaism! Sarah is the free woman, therefore she represents those who are saved apart from the law. Going back to verse 19, Paul describes his ministry as “bearing free children” like Sarah, while the agitators are “bearing slave children” like Hagar. It is all that they are capable of since they are still under the yoke of the Law (Witherington, Galatians, 331).
The point of the analogy is made clear when we realize that Paul is taking on the role of Sarah and commanding that the agitators be expelled from the church! As strange as it sounds, Paul is speaking the words of Sarah to the congregations. The agitators must be removed because there is danger in letting them remain. Like Ishmael, they threaten the (spiritual) life of the true heirs of Abraham.
This seems strong by contemporary standards, but for Paul this is critical to the health of the church. The agitators are attacking what it means to be “in Christ” and therefore risk destroying the church. As he will say in 5:9-10, the bad yeast must be wholly removed and thrown away. Just a little legalism is enough to ruin the whole church!