That means that it is not the natural descendants who automatically inherit the promise, but, on the contrary, that the children of the promise (i.e. sons of God) are to be considered truly Abraham’s children. For it was a promise when God said: ‘At this time I will come and Sarah shall have a son’. (Everybody, remember, thought it quite impossible for Sarah to have a child.) And then, again, a word of promise came to Rebecca, at the time when she was pregnant with two children by the one man, Isaac our forefather. It came before the children were born or had done anything good or bad, plainly showing that God’s act of choice has nothing to do with achievements, good or bad, but is entirely a matter of his will. The promise was: ‘The older shall serve the younger’.
And years later, when this son Isaac was grown up and married and Rebecca his wife was about to bear him twin children, God told her that Esau, the child born first, would be a servant to Jacob, his twin brother. In the words of the Scripture, “I chose to bless Jacob but not Esau.” And God said this before the children were even born, before they had done anything either good or bad. This proves that God was doing what he had decided from the beginning; it was not because of what the children did but because of what God wanted and chose.
And that’s not the only time. To Rebecca, also, a promise was made that took priority over genetics. When she became pregnant by our one-of-a-kind ancestor, Isaac, and her babies were still innocent in the womb—incapable of good or bad—she received a special assurance from God. What God did in this case made it perfectly plain that his purpose is not a hit-or-miss thing dependent on what we do or don’t do, but a sure thing determined by his decision, flowing steadily from his initiative. God told Rebecca, “The firstborn of your twins will take second place.” Later that was turned into a stark epigram: “I loved Jacob; I hated Esau.”
And not only she, but also Rebecca had two sons of one lying-by of Isaac, our father. [Forsooth not only she, but and Rebecca, of one lying-by, or of one knowing of man, having two sons of Isaac, our father.]