(2) (a) After this avowal of his deep sorrow, and his reasons for feeling it, the apostle now proceeds to deal with the subject. First (as has been said above) he shows (Romans 9:6-13) that the present exclusion of the great majority of the Jews from Christian privileges does not imply any unfaithfulness on God's part to his ancient promises; and thus it follows that the fact of their exclusion is no proof of the gospel not being the true fulfilment of those promises.
But it is not as though the Word of God hath taken none effect (or, hath come to naught, ἐκπεπτωκεν). For they are not all Israel who are of Israel: neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called. The promises to the patriarchs never, from the first, implied the inheritance of them by all the physical descendants of those patriarchs; even in Israel there is a recognized distinction between being of the race of Israel and being the true Israel of God; in the original promise to Abraham the descendants of Ishmael (though equally with those of Isaac, his physical seed) were excluded. And so even the race of Israel is but a part of the whole seed of Abraham, to whom the promise was made. Hence it follows that the present exclusion of the majority of even the race of Israel from the inheritance of the promises is not inconsistent with the original purport of those promises. The quotation from Genesis 21:12, "In Isaac," etc., is properly (as in the original Hebrew) "In Isaac shall a seed be named to thee;" i.e. "In Isaac it shall come to pass that posterity of thine shall have the name and position of the seed of Abraham, and be recognized as the inheritors of the promise" (Meyer).
That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for seed. For the word of promise is this, At this time will I come, and Sarah shall have a son (Genesis 18:10). In other words, it is not in virtue of mere carnal descent, but of the promise, that any are so counted; mere carnal descent establishes no claim. It is to be observed that in the first recorded promises to Abraham (Genesis 13:15; Genesis 15:5; Genesis 17:7) there was no restriction; and so through Ishmael, who is also called Abraham's seed (Genesis 21:13), as well as through Isaac, the fulfilment might have been. But the subsequent promise (Genesis 17:19, Genesis 17:21; Genesis 18:10, Genesis 18:14) limited it to Isaac; which limiting promise is, therefore, in Romans 9:9, referred to. With τέκνα τοῦ θεοῦ in Romans 9:8 Compare ἡ υἱοθεσίαα (Romans 9:4), and also Isaiah 63:16. The apostle may have been led to use the expression here in view of the spiritual sonship to God of Christians (cf. Romans 8:15, etc.)which was typified and prepared for by the υἱοθεσία of the chosen seed. A still further limitation of "the children of the promise" is next referred to; and one still more telling for the apostle's argument. It might be said that Ishmael was not, even carnally, the true seed, as being bern, not of the with, but of the bondwoman; or perhaps that he had forfeited any claim he might have had by his proved unworthiness (Genesis 21:9, etc.). But Esau and Jacob were twin children, not only of the same patriarch ( ἐξ ἑνὸς), but also of the same wedded wife; and yet one was chosen and the other rejected, and this even before birth; so that, as the selection was not due to carnal descent, so neither could it be due to proved desert. Thus by this second consideration is disposed of the Jew's assertion of an indefeasible claim to inheritance of the promises on the ground of his boasted works, as by the other is disposed of his claim on the ground of his race. St. Paul's argument to the Jews of his own day would be—You cannot set up a claim to be all of you the necessary inheritors of the promises for all time on the ground either of your carnal descent or of your works, since the selection of Israel himself did not depend on either of these grounds; nor can you say that my position (viz. that Christian believers, to the exclusion of most of you, are now the true inheritors of the promises) implies unfaithfulness in God to his ancient promises; for it is in accordance with the principle on which, according to your own Scriptures, he fulfilled of old his promises to the patriarchs. St. Paul, however, is not to be understood here as writing with a direct polemical intention, but rather as discussing a problem which had at one time perplexed himself, and which seemed to him to call for solution.