John Calvin Commentary Genesis 17:19

"And God said, Nay, but Sarah thy wife shall bear thee a son; and thou shalt call his name Isaac: and I will establish my covenant with him for an everlasting covenant for his seed after him." — Genesis 17:19 (ASV)

Sarah your wife shall bear you a son indeed. Some take the adverb אבל (abal), to mean ‘Truly.’ Others, however, more rightly suppose it is used to increase 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 you from raising yourself higher; and thus it happens that you confine your thoughts within too narrow limits. Now, therefore, enlarge your 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, so that Abraham may from this learn to hope for the blessing previously 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. This was so that he might now more fully rely upon the heavenly oracles and fix the anchor of his faith—which had previously wavered in a fallacious imagination—on the firm truth of God.

He calls the covenant everlasting, in the sense that 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, so 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. Rather, in adopting the whole family of Abraham, he 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 blessing.