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. Yet there is an indirect reference to the calling of the Gentiles, and the rejection of the Jews: for the latter boast of their ancestry, while the former, without any human interference, are become the spiritual offspring of Abraham.

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.

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, על דרך בשר 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 (x) flesh; but he of the freewoman [was] by (y) promise.
(x) As all men are, and by the common course of nature.
(y) By virtue of the promise, which Abraham laid hold on for himself and his true seed, for otherwise Abraham and Sara were past the begetting and bearing of children.

But he who was of the bondwoman,.... Ishmael, who was begotten and born of Hagar,
was born after the flesh; after the common order and course of nature, through the copulation of two persons, the one able to procreate, and the other fit for the conception of children; and was typical of the Jews, the natural descendants of Abraham, who, as such, and upon that account, were not the children of God, nor heirs of the eternal inheritance:
but he of the free woman was by promise; by a previous promise made by God to Abraham, that he should have a son in his old age, when his body was now dead, and when Sarah his wife, who had always been barren, was now grown old, and past the time of bearing children; so that Isaac was born out of the common order and course of nature; his conception and birth were owing to the promise and power of God, and to his free grace and favour to Abraham. This son of promise was a type of the spiritual seed of Abraham, whether Jews or Gentiles, the children of the promise that are counted for the seed; who are born again of the will, power, and grace of God, and are heirs, according to the promise, both of grace and glory, when they that are of the law, and the works of it, are not. All which is further illustrated in the following verses.

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 (Romans 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).

Was born after the flesh - In a natural way. By promise - Through that supernatural strength which was given Abraham in consequence of the promise.