And laughed— If we compare the twelfth verse of the following chapter, where Sarah expresses her incredulity by laughing, and consider the whole sentence here before us (especially the words, Genesis 17:18. O! that Ishmael might live before thee; as if he had said, to have a son by Sarah, in her advanced time of life, is too much for me to expect; it will be enough for me to be assured, that Ishmael shall live and share thy favour); and if, after this, we consider the reply which God makes, repeating the assurance with an emphasis, "Sarah shall bear thee a son INDEED," we shall judge it probable, from all these considerations, that the good old patriarch was a little doubtful, not whether an Omnipotent Being could, but whether he would, accomplish so extraordinary an event: for it is to be observed, that this is the first time that Abraham is directly told that Sarah is to be the mother of the promised seed: and perhaps the decays of age, and the infirmities of nature which he perceived in himself, might make him in some degree apprehend the event more incredible now than it appeared fifteen years before. Thus it is observed of Moses, who probably was not inferior to Abraham in piety and faith, that, after all the divine miracles he had seen, he was guilty of some distrust, and smote the rock twice. Numbers 20:11-12. However, we may observe, as to Abraham, that, if he doubts, it is with great modesty; for he falls upon his face, and checks the risings of unbelief, not daring to speak out what he thought; he only said in his heart, &c.
Be born to him that is an hundred, &c.— See the remarks on chap. Genesis 25:1-2, &c.