John Chrysostom, on Rom 3:27

"Where is boasting then? it is excluded," he says. "By what law? of works? Nay, but by the law of faith." Paul is at great pains to show that faith is mighty to a degree which was never even fancied of the Law. For after he had said that God justifieth man by faith, he grapples with the Law again. And he does not say, where then are the well doings. of the Jews? where their righteous dealing? but, "where is then the boasting?" so taking every opportunity of showing, that they do but use great words, as though they had somewhat more than others, and have no work to show. And after saying, "Where then is the boasting?" he does not say, it is put out of sight and hath come to an end, but "it is excluded," which word rather expresses unseasonableness; since the reason for it is no more. For as when the judgment is come they that would repent have not any longer the season for it, thus now the sentence being henceforth passed, and all being upon the point of perishing, and He being at hand Who by grace would break these terrors, they had no longer the season for making a plea of amelioration wrought by the Law. See he calls the faith also a law delighting to keep to the names, and so allay the seeming novelty. But what is the "law of faith?" It is, being saved by grace. Here he shows God's power, in that He has not only saved, but has even justified, and led them to boasting, and this too without needing works, but looking for faith only.