Commentaries
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.
23. after the flesh—born according to the usual course of nature: in contrast to Isaac, who was born "by virtue of the promise" (so the Greek), as the efficient cause of Sarah's becoming pregnant out of the course of nature (Ro 4:19). Abraham was to lay aside all confidence in the flesh (after which Ishmael was born), and to live by faith alone in the promise (according to which Isaac was miraculously born, contrary to all calculations of flesh and blood).