Was born after the flesh - Ishmael was born according to the course of nature, his parents being both of a proper age, so that there was nothing uncommon or supernatural in his birth: this is the proper meaning of the apostle's κατα σαρκα, after or according to the flesh, and answers to the Hebrew phrase, בשר דרך על al derec basar, according to the manner of the flesh, i.e. naturally, according to the common process of nature.
By promise - Both Abraham and Sarah had passed that age in which the procreation of children was possible on natural principles. The birth, therefore, of Isaac was supernatural; it was the effect of an especial promise of God; and it was only on the ground of that promise that it was either credible or possible.
But he who was of the bondwoman was born after the flesh - In the ordinary course of nature, without any special promise, or any unusual divine interposition, as in the case of Isaac.
But he of the free woman - The birth of Isaac was in accordance with a special promise, and by a remarkable divine interposition; see Genesis 18:10; Genesis 21:1-2; Hebrews 11:11-12; compare the notes at Romans 4:19-21. The idea here of Paul is, that the son of the slave was in a humble and inferior condition from his very birth. There was no special promise attending him. He was born into a state of inferiority and servitude which attended him through his whole life. Isaac, however, was met with promises as soon as he was born, and was under the benefit of those promises as long as he lived. The object of Paul is, to state the truth in regard to a condition of servitude and slavery. It is attended with evils from beginning to end; from the birth to the grave. By this illustration he means to show them the folly of becoming the voluntary slaves of the Law after they had once been made free.