21 Say, you whose desire it is to be under the law, do you not give ear to the law? 22 Because it is in the Writings, that Abraham had two sons, one by the servant-woman, and one by the free woman. 23 Now the son by the servant-woman has his birth after the flesh; but the son by the free woman has his birth through the undertaking of God. 24 Which things have a secret sense; because these women are the two agreements; one from the mountain of Sinai, giving birth to servants, which is Hagar. 25 Now this Hagar is the mountain Sinai in Arabia, and is the image of the Jerusalem which now is: which is a servant with her children. 26 But the Jerusalem on high is free, which 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.