Chapter 13 [VIII.]— Keeping the Law; The Jews' Glorying; The Fear of Punishment; The Circumcision of the Heart
Then comes what I mentioned above; then he shows what the Jew is, and says that he is called a Jew, but by no means fulfils what he promises to do. “But if,” says he, “you call yourself a Jew, and restest in the law, and makest your boast of God, and know His will, and triest the things that are different, being instructed out of the law; and art confident that you are yourself a guide of the blind, a light of them that are in darkness, an instructor of the foolish, a teacher of babes, which hast the form of knowledge and of the truth in the law.
You therefore who teaches another, do you not teach yourself? You that preaches a man should not steal, do you steal? You that says a man should not commit adultery, do you commit adultery? You that abhors idols, do you commit sacrilege? You that makes your boast of the law, through breaking the law do you dishonor God? For the name of God is blasphemed among the Gentiles through you, as it is written. Circumcision verily profits, if you keep the law; but if you be a breaker of the law, your circumcision is made uncircumcision.
Therefore, if the uncircumcision keep the righteousness of the law, shall not his uncircumcision be counted for circumcision? And shall not uncircumcision which is by nature, if it fulfil the law, judge you, who by the letter and circumcision dost transgress the law? For he is not a Jew who is one outwardly; neither is that circumcision which is outward in the flesh: but he is a Jew who is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.” Here he plainly showed in what sense he said, “You make your boast of God.”
For undoubtedly if one who was truly a Jew made his boast of God in the way which grace demands (which is bestowed not for merit of works, but gratuitously), then his praise would be of God, and not of men. But they, in fact, were making their boast of God, as if they alone had deserved to receive His law, as the Psalmist said: “He did not the like to any nation, nor His judgments has He displayed to them.” And yet, they thought they were fulfilling the law of God by their righteousness, when they were rather breakers of it all the while!
Accordingly, it “wrought wrath” upon them, and sin abounded, committed as it was by them who knew the law. For whoever did even what the law commanded, without the assistance of the Spirit of grace, acted through fear of punishment, not from love of righteousness, and hence in the sight of God that was not in the will, which in the sight of men appeared in the work; and such doers of the law were held rather guilty of that which God knew they would have preferred to commit, if only it had been possible with impunity.
He calls, however, “the circumcision of the heart” the will that is pure from all unlawful desire; which comes not from the letter, inculcating and threatening, but from the Spirit, assisting and healing. Such doers of the law have their praise therefore, not of men but of God, who by His grace provides the grounds on which they receive praise, of whom it is said, “My soul shall make her boast of the Lord;” and to whom it is said, “My praise shall be of You:” but those are not such who would have God praised because they are men; but themselves, because they are righteous.
Source: On the Spirit and the Letter (New Advent)