The birth of Isaac was a thing extraordinary; for which, neither Abraham nor Sarah had any ground to hope, but only that promise made, Genesis 18:10, in these words: At this time, or according to this time, i.e. the time of bearing children after conception, will I come, and Sarah shall have a son; i.e. I will manifest my power in fulfilling my promise of giving thee a son. By which it is clear, that the birth of Isaac was an effect of God's promise, and nothing else. So that they to whom the promise belong, (whosoever they be), they are the seed of Abraham, and, upon the obedience of faith, shall be accepted for the children of God. The apostle Peter tells women, (whether Jews or Gentiles, it matters not), that by well-doing they become the daughters of Sarah.
Related Commentaries of Romans 9:9
Romans 9:9
9 For this is the word of promise, At this time will I come, and Sara shall have a son.