His son by the slave woman was born according to the flesh, but his son by the free woman was born through the promise.
23. But he who was of the bond woman. Both were sons of Abraham according to the flesh; but in Isaac there was this peculiarity, that he had the promise of grace. In Ishmael there was nothing besides nature; in Isaac there was the election of God, signified in part by the manner of his birth, which was not in the ordinary course, but miraculous.
In these verses the apostle illustrates the difference between believers who rested in Christ only and those judaizers who trusted in the law, by a comparison taken from the story of Isaac and Ishmael. This he introduces in such a manner as was proper to strike and impress their minds, and to convince them of their great weakness in departing from the truth, and suffering...
Commenting on Galatians 4:21-31
Which things are an allegory,.... Or "are allegorized": so Sarah and Hagar were allegorized by Philo the Jew (p), before they were by the apostle. Sarah he makes to signify virtue, and Hagar the whole circle of arts and sciences, which are, or should be, an handmaid to virtue; but these things respecting Hagar and Sarah, the bondwoman and the free, and their several offspring...