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.
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.”
The strangeness of the allegory to modern thinking is due to the fact that we do not think in biblical terms as Paul did. He is neither arbitrary nor fanciful in finding an allegorical significance in the story of Hagar and Sarah, for in fact it illustrates a fundamental principle in God's dealings with men, which comes up again and again in Paul's writings, namely, the promise. Divine, electing grace, according to promise - this is what the whole story of the Bible is about. The great Antitype is seen in the New Testament, and the Old Testament gives pointers towards it, enacting the drama on a small scale, so to speak, that was to fill the world-stage in the fullness of the time. When this is appreciated, then it is obvious that we should expect drama all through the period that was preparatory to its full manifestation in history. This is why, rightly considered and rightly understood, this story from the book of Genesis is absolutely authentic and relevant for Paul's purpose.