“For this is the word of promise, At this time will I come, and Sara shall have a son.”
King James Version (KJV)
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|.
Ro 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.