Romans 2:18

"But if you are called a Jew, and rest in the law, and glory in God, and know his will, and approve the more useful things, being instructed by the law, and are confident that you are a guide of the blind, a light of those who are in darkness, an instructor of the foolish, a teacher of infants, having the form of knowledge and of truth in the law. You then who teach another, do you not teach yourself? You who preach that a man should not steal, do you steal?

For they become ridiculous even to the Gentiles themselves when, following the letter of the Law, they say: "Do not touch, do not taste, do not handle: all of which things are unto corruption according to the commandments and doctrines of men." But these things the Apostle indeed says to him who is a Jew in name, not in deed; yet they can pertain also to every man in whom there is only the name of religion and piety, while works, and knowledge, and faith are lacking. And therefore we ought to examine these matters with greater care in ourselves than in those who do not come to faith in Christ, lest any of us who through faith in Christ and through the circumcision given in baptism has been made a true Jew, and rests in the law of Christ, and boasts also that, having driven off the error of idols, he has come to know God and that His will has become known to him (for he knows how to prove what is good, and well-pleasing, and perfect, and has advanced so far that he is even a leader and teacher of the Church for enlightening those who are blind in knowledge, and for instructing the little ones in Christ) — lest, I say, such a man be willing indeed to teach others quite strictly, and to demand from those whom he instructs the highest discipline of chastity, while he himself is driven by the vice of intemperance and of covetousness, and sometimes even burns with the flames of hidden lust. To a man of this sort, then, this apostolic word is fittingly applied, saying to him: "You who teach another, do you not teach yourself? You who preach that a man should not steal, do you steal? […]

"Behold, thou art called a Jew, and restest in the Law, and makest thy boast of God, and knowest His will, and approvest the things that are more excellent, being instructed out of the Law." After saying that the Gentile wanteth nothing appertaining to salvation if he be a doer of the Law, and after making that wonderful comparison, he goes on to set down the glories of the Jews, owing to which they thought scorn of the Gentiles: and first the very name itself, which was of great majesty, as the name Christian is now. For even then the distinction which the appellation made was great. And so he begins from this, and see how he takes it down. For he does not say, Behold, thou art a Jew, but "art called" so, "and makest thy boast in God;" that is, as being loved by Him, and honored above all other men. And here he seems to me to be gently mocking their unreasonableness, and great madness after glory, because they misused this gift not to their own salvation, but to set themselves up against the rest of mankind, and to despise them. "And knowest His will, and approvest the things that are more excellent." Indeed this is a disadvantage, if without working: yet still it seemed to be an advantage, and so he states it with accuracy. For he does not say, thou doest, but knowest; and approvest, not followest and doest.

It is hardly surprising that a Jew should believe, since he has been taught to do so by the law. It is indeed dangerous for him not to believe if he has the law as his guide.

"And know His will," that is, God's. "And approve the things that are excellent," that is, you determine what must be done and what must not be done. By "excellent" (διαφέροντα) one should understand what is fitting or beneficial for each person.