Romans 9:6-7 — Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:
Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
Paul repeats his point from Romans 2:28-29. He makes a distinction between Jews by birth (natural descent) and those who believe the word of God (promise to Abraham).
Called = chosen
Abraham had at least two sons, Ishmael and Isaac. The Jew would say the physical descent from Abraham makes him an Israelite. He declared any man born a Jew had a right to the promises of God. In contradistinction to this, Paul demonstrates that God never worked on that principle. What about Abraham himself? Though Ishmael was a son by physical birth, he is not an Israelite. God set Ishmael aside and chose Isaac instead. As he says, "in Isaac shall thy seed by called." Here is a clear instance of election.
But someone might say, "Ishmael was not legitimate. He was born of a handmaid," which was true.So Paul takes the case of Jacob. What are the facts in this case? Two boys were born; they were twins. Jacob and Esau were twins; so that the Jew could not say, "Well, the one man had the prior right because he was the older." Here were two sons with the same father and the same mother, born at the same time. Not only so, but Esau was born first, and if there should be any priority on the ground of age, Esau should have had the priority. What did God do? He said, "The Older shall serve the younger." Every Jew knew that. This was another instance of election, God working in absolute sovereignty (verse 11).
That is the answer to the Jew. God chooses according to His own sovereign will and sets aside all human ideas of merit and superiority. — Romans: The Gospel of God's Grace, by Alvin J. McClain, page 180.