21 Tell me, you that desire to be under the law, don’t you listen to the law? 22 For it is written that Abraham had two sons, one by the handmaid, and one by the free woman. 23 However, the son by the handmaid was born according to the flesh, but the son by the free woman was born through promise. 24 These things contain an allegory, for these are two covenants. One is from Mount Sinai, bearing children to bondage, which is Hagar. 25 For this Hagar is Mount Sinai in Arabia, and answers to the Jerusalem that exists now, for she is in bondage with her children. 26 But the Jerusalem that is above is free, which is the mother of us all.
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.