Romans 9:6-9

Well then, has God failed to fulfill his promise to Israel? No, for not all who are born into the nation of Israel are truly members of God’s people! Being descendants of Abraham doesn’t make them truly Abraham’s children. For the Scriptures say, “Isaac is the son through whom your descendants will be counted,” though Abraham had other children, too. This means that Abraham’s physical descendants are not necessarily children of God. Only the children of the promise are considered to be Abraham’s children. For God had promised, “I will return about this time next year, and Sarah will have a son.”