21 Tell me, you who desire to be under law, do you not hear the law? 22 For it is written that Abraham had two sons, one by a slave and one by a free woman. 23 But the son of the slave was born according to the flesh, the son of the free woman through promise. 24 Now this is an allegory: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar. 25 Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children. 26 But the Jerusalem above is free, and she is our mother. 27 For it is written, "Rejoice, O barren one who does not bear; break forth and shout, you who are not in travail; for the children of the desolate one are many more than the children of her that is married."
Matthew Henry's Commentary on Galatians 4:21-27
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.