Romans 9:8

f. τοῦτʼ ἔστιν : the meaning of this action of God is now made clear. It signifies that not mere bodily descent from Abraham makes one a child of God that was never the case, not even in Abraham's time; it is the children of the promise who are reckoned a seed to Abraham, for the word in virtue of which Isaac, the true son and heir, was born, was a word of promise. He was born, to use the language of the Gospel, from above; and something analogous to this is necessary, whenever a man (even a descendant of Abraham) claims to be a child of God and an heir of His kingdom. From Galatians 4:28 (Now we, brethren, like Isaac, are children of promise) we see that the relation to God in question here is one open to Gentiles as well as Jews: if we are Christ's, then we too are Abraham's seed, and heirs according to promise. The argumentative suggestion in Romans 9:6-9 is that just as God discriminated at the first between the children of Abraham, so He is discriminating still; the fact that many do not receive the Gospel no more proves that the promise has failed than the fact that God chose Isaac only and set aside Ishmael.