7. And they are not all the children of Abraham because they are by blood his descendants. No, [the promise was] Your descendants will be called and counted through the line of Isaac [though Abraham had an older son]. [Gen. 21:9-12.]
8. That is to say, it is not the children of the body [of Abraham] who are made God's children, but it is the offspring to whom the promise applies that shall be counted [as Abraham's true] descendants.
9. For this is what the promise said, About this time [next year] will I return and Sarah shall have a son. [Gen. 18:10.]