Augustine of Hippo : ISHMAEL, CHILD OF THE FLESH, ISAAC, CHILD OF THE PROMISE. — It should be noted that Ishmael is also called by God the descendant of Abraham, according to the words, which, according to the Apostle, must be taken in this regard: Isaac will be your offspring; (Gen. 21:12) That is, they are counted as offspring, not the children according to the flesh, but the children according to the promise (Rom. 9:7-8). So that this, properly speaking, is quite fitting for Isaac, who was not a son according to the flesh, but a son according to the promise, when that promise is made with reference to all peoples. [ — QUESTIONS ON GENESIS, Q. 51 (tr. J. Litteral, draft)