5. To them belong the patriarchs, and as far as His natural descent was concerned, from them is the Christ, Who is exalted and supreme over all, God, blessed forever! Amen (so let it be).
6. However, it is not as though God's Word had failed [coming to nothing]. For it is not everybody who is a descendant of Jacob (Israel) who belongs to [the true] Israel.
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.