Romans 9:8

That is, it is not the children of the flesh [of Abraham] that are [reckoned or accounted as] children of God; but the children of the promise are reckoned for a seed. [Are accounted the children of God through Abraham. Fleshly descent from Abraham, of itself and without more--i. e., without promise--never availed for any spiritual blessing (Galatians 4:23). "This," says Trapp, "profiteth them no more than it did Dives, that Abraham called him son" (Luke 16:25). So flesh avails neither then nor now, but promise. Paul proceeds to show that Isaac was a son of promise, and whatever covenants or promises availed for his children came to them because they, through him, became symbolically sons of promise, Isaac typifying Christ, the real son of promise given to Abraham (Galatians 3:16), and Isaac's posterity typifying the real children of promise, the regenerated sons of God begotten unto Christ through the gospel (Galatians 4:28; John 1:12-13). So as Abraham had a fleshly seed according to the first promise, "In Isaac shall thy seed be called," these being Jews; so he had a spiritual seed according to the second promise, "In thee and in thy seed shall all the nations (Gentiles; but not excluding Jews) of the earth be blessed," these being Gentiles. Hence, if the two promises were each kept with the two parties to whom they were severally given, the word of God was not broken, and his promise had not failed. But such was indeed the case, for God kept his word with the fleshly seed, fulfilling to them the fleshly promise that Christ should be born of their stock (John 4:22; Galatians 3:16), and to the spiritual seed he was fulfilling the spiritual promise granting them eternal life through that faith in Christ which made them spiritual children of Abraham, the father of the faithful (Galatians 3:7-14). So it was not two promises to one seed, but two promises to two seeds, and each promise was kept of God to each promisee. And why, says Paul, do we call Isaac the son of promise? Because he was not born according to the natural law of the flesh, his mother being past bearing, but contrary to nature and by reason of the divine power, working to fulfill the promise of God, which promise is as follows]