21 Tell me, you who want to live under the law, do you know what the law actually says? 22 The Scriptures say that Abraham had two sons, one from his slave wife and one from his freeborn wife. 23 The son of the slave wife was born in a human attempt to bring about the fulfillment of God's promise. But the son of the freeborn wife was born as God's own fulfillment of his promise. 24 These two women serve as an illustration of God's two covenants. The first woman, Hagar, represents Mount Sinai where people received the law that enslaved them. 25 And now Jerusalem is just like Mount Sinai in Arabia, because she and her children live in slavery to the law. 26 But the other woman, Sarah, represents the heavenly Jerusalem. She is the free woman, and she is our mother.
Matthew Henry's Commentary on Galatians 4:21-26
Commentary on Galatians 4:21-27
(Read Galatians 4:21-27)
The difference between believers who rested in Christ only, and those who trusted in the law, is explained by the histories of Isaac and Ishmael. These things are an allegory, wherein, beside the literal and historical sense of the words, the Spirit of God points out something further. Hagar and Sarah were apt emblems of the two different dispensations of the covenant. The heavenly Jerusalem, the true church from above, represented by Sarah, is in a state of freedom, and is the mother of all believers, who are born of the Holy Spirit. They were by regeneration and true faith, made a part of the true seed of Abraham, according to the promise made to him.