So himself offering up thanksgiving for all men unto the Only-Begotten of God. What, he says, if others do blaspheme? Still we who know His mysteries, and His unspeakable Wisdom, and great Providence over us, know well that it is not to be blasphemed, but to be glorified, that He is worthy. Still not satisfied with being himself conscious of it, he endeavors next to use arguments, and to use a sharper way of speech against them. And he does not direct his aim at them, without first divesting them of a suspicion they had. Lest then he should seem to be addressing them as enemies, further on he says “Brethren, my heart's desire and prayer to God for Israel is, that they might be saved.” And here, along with other remarks, he so ordered things, as not to seem to be saying what he was going to say out of enmity against them. Hence he does not decline calling them even kinsmen and brothers. For even if it was for Christ's sake that he said what he did, still he is for drawing (ἐ πισπἅται) their mind to him also, and paves his way to what he has to say, and quits himself of all suspicion owing to what had to be said against them, and then he at last goes into the subject most of them were looking for. For many, as I have already stated, wanted to know what was the reason why they who had received the promise fell short of it, while those who had even never heard of it were saved before them. Therefore, to clear up this difficulty, he brings forward the answer before the objection. For to prevent any from saying, What? Are you more thoughtful for God's glory than God is for His own? And does He need your aid that His word may not fall to the ground? In reply to these things he says, I spoke this not as if God's Word had fallen to the ground, but to show my love for Christ. For as things have had this issue, we are in no want of words in God's behalf, or of showing that stand His promise did. God said to Abraham, “To you and to your seed will I give the land.” And, “In your seed shall all the nations of the earth be blessed.” Let us see then, he says, of what sort this seed is. For it is not all that are from him that are his seed. Whence he says, “For they are not all Israel that are of (or from) Israel.”
Ver. 7. “Neither, because they are the seed of Abraham, are they all children.”
Now when you come to know of what kind the seed of Abraham is, you will see that the promise is given to his seed, and know that the word has not fallen to the ground. Of what kind, pray, is the seed then? It is no saying of mine, he means, but the Old Testament itself explains itself by saying as follows, “In Isaac shall your seed be called.” What is, “In Isaac?” Explain.
Ver. 8. “That is, they which are the children of the flesh, these are not the children of God: but the children of the promise, these are counted for the seed.”
And observe the judgment and depth of Paul's mind. For in interpreting, he does not say, “they which are the children of the flesh, these are not” the children of Abraham, but, “the children of God:” so blending the former things with the present, and showing that even Isaac was not merely Abraham's son. And what he means is something of this sort: as many as have been born as Isaac was, they are sons of God, and of the seed of Abraham. And this is why he said, “in Isaac shall your seed be called.” That one may learn that they who are born after the fashion of Isaac, these are in the truest sense Abraham's children. In what way was Isaac born then? Not according to the law of nature, not according to the power of the flesh, but according to the power of the promise. What is meant then by the power of “the promise?”
Ver. 9. “At this time will I come, and Sarah shall have a son.”
This promise then and word of God it was that fashioned Isaac, and begot him. For what if a womb was its instrument and the belly of a woman? Since it was not the power of the belly, but the might of the promise that begot the child. Thus are we also gendered by the words of God. Since in the pool of water it is the words of God which generate and fashion us. For it is by being baptized into the Name of the Father and of the Son and of the Holy Ghost that we are gendered. And this birth is not of nature, but of the promise of God. For as after first foretelling the birth of Isaac, He then accomplished it; so ours also He had announced before, many ages ago by all the Prophets, and afterwards brought it to pass. You know how great He has set it forth as being, and how, as He promised a great thing, He furnished it with abundant ease! But if the Jews were to say, that the words, “In Isaac shall your seed be called,” mean this, that those born of Isaac should be reckoned to him for a seed, then the Edomites too, and all those people, ought to be denominated his sons, since their forefather Esau was a son of his. But now so far are they from being called sons, that they are the greatest possible aliens. You see then that it is not the children of the flesh that are the children of God, but that even in nature itself the generation by means of baptism from above was sketched out beforehand. And if you tell me of the womb, I in return have to tell you of the water. But as in this case all is of the Spirit, so in the other all was of promise. For the womb was more chilled than any water owing to barrenness and to old age. Let us then gain accurate knowledge of our own nobility, and display a life worthy of it. For in it is nothing fleshly or earthy: hence neither let there be in us. For it was neither sleep, nor the will of the flesh, nor embraces, nor the madness of desire, but “God's love toward man,” which wrought the whole. And as in that case it was when the age was past hope, so in this also it was when the old age of sins had come over us, that Isaac suddenly sprang up in youth, and we all became the children of God, and the seed of Abraham.
Ver. 10. “And not only this; but when Rebecca also had conceived by one, even by our father Isaac.”
Source: Homilies on Romans (New Advent)