Genesis 17:19

19.Sarah thy wife shall bear thee a son indeed Some take the adverb אבל (abal,) to mean ‘Truly.’ Others, however, more rightly suppose it to be used for increasing the force of the expression. For God rouses the slumbering mind of his servant; as if he would say, ‘The sight of one favor prevents thee from raising thyself higher; and thus it happens that thou dost confine thy thoughts within too narrow limits. Now, therefore, enlarge thy mind, to receive also what I promise concerning Sarah. For the door of hope ought to be sufficiently open to admit the word in its full magnitude.’

And I will establish my covenant with him He confines the spiritual covenant to one family, in order that Abraham may hence learn to hope for the blessing before promised; for since he had framed for himself a false hope, not founded on the word of God, it was necessary that this false hope should first be dislodged from his heart, in order that he might now the more fully rely upon the heavenly oracles, and might fix the anchor of his faith, which before had wavered in a fallacious imagination, on the firm truth of God. He calls the covenant everlasting, in the sense which we have previously explained. He then declares that it shall not be bound to one person only, but shall be common to his whole race, that it may, by continual succession, descend to his posterity. Yet it may seem absurd, that God should command Ishmael, whom he deprives of his grace, to be circumcised. I answer; although the Lord constitutes Isaac the firstborn and the head, from whom he intends the covenant of salvation to flow, he still does not entirely exclude Ishmael, but rather, in adopting the whole family of Abraham, joins Ishmael to his brother Isaac as an inferior member, until Ishmael cut himself off from his father’s house, and his brother’s society. Therefore his circumcision was not useless, until he apostatized from the covenant: for although it was not deposited with him, he might, nevertheless, participate in it, with his brother Isaac. In short, the Lord intends nothing else, by these words, than that Isaac should be the legitimate heir of the promised benediction.