And not only this
This instance of Ishmael and Isaac, is not the only one, proving that Abraham's natural seed, the children of the flesh, are not all children, the children of God:
but when Rebecca also had conceived by one, even by our
"it was said unto her", ( Romans 9:12 ) , being in a parenthesis, "the elder shall serve the younger". The apostle was aware, that the Jews would be ready to say, that the instance of Ishmael and Isaac was not a pertinent one; since Ishmael was not born of Sarah, the lawful wife of Abraham, but of a bondwoman, which was the reason his rejection, when Isaac was the son of promise, by the lawful wife, and that they were children of Abraham in the line of Isaac, and so children of the promise, as he was: wherefore he proceeds to mention the case of Jacob and Esau, which was not liable to any such exception; seeing they not only had the same father, but the same mother, Isaac's lawful wife; they were conceived by Rebecca at once, were in the same womb together, were twins, and if any had the preference and advantage, Esau had it, being born first; and yet a difference was made between these two by God himself, and which was notified by him to the mother of them, before either were born.