{6} For this is the word of promise, At this time will I come, and Sara shall have a son.

(6) A reason of that application: because Isaac was born by the power of the promise, and therefore he was not chosen, no, he was not at all, except by the free will of God: by which it follows that the promise is the fountain of predestination, and not the flesh, from which promise the particular election proceeds, that is, that the elect are born elect, and not that they are first born, and then after elected, by God who predestinates.

People's New Testament

9:9 This is the word of promise. This promise was made when Sarah was far beyond the natural age of bearing children, and when Abraham was an old man. So the chosen seed are children of promise.

Wesley's Notes

9:9 For this is the word of the promise - By the power of which Isaac was conceived, and not by the power of nature. Not, Whosoever is born of thee shall be blessed, but, At this time - Which I now appoint. I will come, and Sarah shall have a son - And he shall inherit the blessing. Gen 18:10.

Matthew Henry's Concise Commentary

9:6-13 The rejection of the Jews by the gospel dispensation, did not break God's promise to the patriarchs. The promises and threatenings shall be fulfilled. Grace does not run in the blood; nor are saving benefits always found with outward church privileges. Not only some of Abraham's seed were chosen, and others not, but God therein wrought according to the counsel of his own will. God foresaw both Esau and Jacob as born in sin, by nature children of wrath even as others. If left to themselves they would have continued in sin through life; but for wise and holy reasons, not made known to us, he purposed to change Jacob's heart, and to leave Esau to his perverseness. This instance of Esau and Jacob throws light upon the Divine conduct to the fallen race of man. The whole Scripture shows the difference between the professed Christian and the real believer. Outward privileges are bestowed on many who are not the children of God. There is, however, full encouragement to diligent use of the means of grace which God has appointed.