and know his will and approve what is excellent, because you are instructed from the law;
18. And knowest his will, and approvest things excellent, etc. He now concedes to them the knowledge of the divine will, and the approval of things useful; and this they had attained from the doctrine of the law. But there is a twofold approval, — one of choice, when we embrace the good we approve; the other of judgment, by which indeed we distinguish good...
In the latter part of the chapter the apostle directs his discourse more closely to the Jews, and shows what sins they were guilty of, notwithstanding their profession and vain pretensions. He had said (Rom 2:13) that not the hearers but the doers of the law are justified; and he here applies that great truth to the Jews. Observe, I.
Commenting on Romans 2:17-29
An instructor of the foolish,.... All men in a state of unregeneracy are foolish, and need instruction; here the common people among the Jews, who knew not the law, are meant; it was very usual to call anyone of their doctors and masters "an instructor": it follows, a teacher of babes; either in age or in understanding; such who taught either of them were called...