Romans 9:9 For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.” (10) And not only so, but also when Rebekah had conceived children by one man, our forefather Isaac, (11) though they were not yet born and had done nothing either good or bad—in order that God's purpose of election might continue, not because of works but because of him who calls— (12) she was told, “The older will serve the younger.” (13) As it is written, “Jacob I loved, but Esau I hated.”
From this passage it is apparent that God foreordained and caused Sarah to have a child at age ninety. It is also true that God’s purpose in election was and is implemented by his sovereign decree. While the twins were still in the womb, God ordained that one of Abraham’s physical grandchildren would be saved and one would not. Thus, Paul drives home the point with Abraham’s children and grandchildren that the Abrahamic Covenant of Promise is a covenant made with the spiritual descendants of Abraham and not his physical descendants. Therefore, there were no heirs to the Abrahamic covenant of promise in the Old Testament era based on physical birth and there are no heirs to the New Covenant in the New Testament era based on physical birth.
Focus on Christ #138