But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. 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.