He had shown that faith is necessary, that it is older than circumcision, that it is more mighty than the Law, that it establishes the Law. For if all sinned, it was necessary: if one being uncircumcised was justified, it is older: if the knowledge of sin is by the Law and yet it was without the Law made evident, it is more mighty: if it has testimony borne to it by the Law, and establishes the Law, it is not opposed to it, but friendly and allied to it. Again, he shows upon other grounds too that it was not even possible by the Law to attain to the inheritance, and after having matched it with the circumcision, and gained it the victory, he brings it besides into contrast with the Law in these words, “For if they which are of the Law be heirs, faith is made void.” To prevent them anyone from saying that one may have faith and also keep up the Law, he shows this to be impracticable. For he that clings to the Law, as if of saving force, does disparagement to faith's power; and so he says, “faith is made void,” that is, there is no need of salvation by grace. For then it cannot show forth its own proper power; “and the promise is made of none effect.” This is because the Jew might say, What need have I of faith? If then this held, the things that were promised, would be taken away along with faith. See how in all points he combats with them from the early times and from the Patriarch. For having shown from thence that righteousness and faith went together in the inheritance, he now shows that the promise did likewise. For to prevent the Jew from saying, What matters it to me if Abraham was justified by faith? Paul says, neither can what you are interested with, the promise of the inheritance, come into effect apart from it: which was what scared them most. But what promise is he speaking of? That of his being “the heir of the world,” and that in him all should be blessed. And how does he say that this promise is made of none effect?
Ver. 15. “Because the Law works wrath: for where no Law is, there is no transgression.”
Now if it works wrath, and renders them liable for transgression, it is plain that it makes them so to a curse also. But they that are liable under a curse, and punishments, and transgression, are not worthy of inheriting, but of being punished and rejected. What then happens? faith comes, drawing on it the grace, so that the promise comes into effect. For where grace is, there is a remitting, and where remitting is, there is no punishment. Punishment then being removed, and righteousness succeeding from faith, there is no obstacle to our becoming heirs of the promise.
Ver. 16. “Therefore it is of faith,” he says, “that it might be by grace; to the end the promise might be sure to all the seed.”
You see that it is not the Law only that faith establishes, but the promise of God also that it will not allow to fall to the ground. But the Law, on the other hand, by being kept to unseasonably, makes even the faith of none effect, and hinders the promise. By this he shows that faith, so far from being superfluous, is even necessary to that degree, that without it there is no being saved. For the Law works wrath, as all have transgressed it. But this does not even suffer wrath to arise at all: for “where no Law is,” he says, “there is no transgression.” Do you see how he not only does away with sin after it has existed, but does not even allow it to be produced? And this is why he says “by grace.” For what end? Not with a view to their being put to shame, but to the end that the promise might be sure to all the seed. Here he lays down two blessings, both that the things given are sure, and also that they are to all the seed, so gathering in those of the Gentiles, and showing that the Jews are without, if they contend against the faith. For this is a surer thing than that. For faith does you no hurt (be not contentious), but even now you are in danger from the Law, it preserves you. Next having said, “to all the seed,” he defines what seed he means. That which is of faith, he says, so blending with it their relationship to the Gentiles, and showing that they must not be proud of Abraham who do not believe as he did. And see a third thing which faith effected besides. It makes the relationship to that righteous man more definite (ἀ κρι βεστέραν), and holds him up as the ancestor of a more numerous issue. And this is why he does not say merely Abraham, but “our father,” ours who believe. Then he also seals what he has said by the testimony—
Ver. 17. “As it is written,” he says, “I have made you a father of many nations.”
Do you observe that this was ordered by Providence from of old? What then, he means, does He say this on account of the Ishmaelites, or of the Amalekites, or of the Hagarenes? This however, as he goes on he proves more distinctly not to be said of these. But as yet he presses forward to another point, by which means he proves this very thing by defining the mode of the relationship, and establishing it with a vast reach of mind. What then does he say?
“Before (or, answering to, κατέναντι) Him Whom he believed, even God.”
But his meaning is something of this sort, as God is not the God of a part, but the Father of all, so is he also. And again, as God is a father not by way of the relationship of nature, but by way of the affiance of faith, so is he also inasmuch as it is obedience that makes him father of us all. For since they thought nothing of this relationship, as clinging to that grosser one, he shows that this is the truer relationship by lifting his discourse up to God. And along with this he makes it plain that this was the reward of faith that he received. Consequently, if it were not so, and he were the father of all the dwellers upon earth, the expression before (or answering to) would be out of place, while the gift of God would be curtailed. For the “before,” is equivalent to “alike with.” Since where is the marvel, pray, in a man's being the father of those sprung from himself? This is what is every man's lot. But the extraordinary thing is, that those whom by nature he had not, them he received by the gift of God. And so if you would believe that the patriarch was honored, believe that he is the father of all. But after saying, “before Him Whom he believed, even God,” he does not pause here, but goes on thus; “Who quickens the dead, and calls those things which be not as though they were,” so laying beforehand his foundations for discoursing upon the resurrection. And it was serviceable also to his present purpose. For if He could “quicken the dead” and bring in “those things that were not as though they were,” then could He also make those who were not born of him to be his children. And this is why he does not say, bringing in the things which are not, but calling them, so showing the greater ease of it. For as it is easy to us to call the things which are by name, so to Him it is easy, yea, and much easier to give a subsistence to things that are not. But after saying, that the gift of God was great and unspeakable, and having discoursed concerning His power, he shows farther that Abraham's faith was deserving of the gift, that you may not suppose him to have been honored without reason. And after raising the attention of his hearers to prevent the Jew from clamoring and making doubts, and saying, “And how is it possible for those who are not children to become children?” he passes on to speak of the patriarch, and says,
Ver. 18. “Who against hope believed in hope, that he might become the father of many nations, according to that which was spoken, So shall your seed be.”
Source: Homilies on Romans (New Advent)