21 Tell me, you who desire to be subject to the law, will you not listen to the law? 22 For it is written that Abraham had two sons, one by a slave woman and the other by a free woman. 23 One, the child of the slave, was born according to the flesh; the other, the child of the free woman, was born through the promise. 24 Now this is an allegory: these women are two covenants. One woman, in fact, is Hagar, from Mount Sinai, bearing children for slavery. 25 Now Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. 26 But the other woman corresponds to the Jerusalem above; she is free, 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.