He did not say, 'You fulfill,' but, 'You understand,' without following it and without doing it."
"And approve what is better," δοκιμάζεις τά διαφέροντα. Δοκιμαζειν means to test, to know by experience, to discern and be assured.
Διαφέροντα means what stands out from the ordinary, what is better and excellent. If understood generally, it means: "You distinguish things opposed to one another: righteousness and unrighteousness, chastity and impurity, piety and impiety" (blessed Theodoret).
But since δοκιμαζειν points to a particular activity, or to its particular trials, these words more likely indicate something else.
The Jew could well determine what, when, where, and under what circumstances he ought to do in order to please God. He could choose the better manner of acting.
That he understood what ought to be done in general is indicated by the words, "you know His will." By the words, "you approve what is better," it is indicated that he could also decide what was better for him to do in particular cases.
Blessed Theophylact writes: "By 'what is better' one must understand what is fitting or useful for each person. By 'you approve' he means: you decide what should be done and what should not be done in each case."
"Being instructed out of the law." "For in all this, it has become your teacher" (blessed Theodoret).
"Being instructed," κατηχούμενος, means being catechized. In the synagogues the law was read, and through the hearing of the ear each person, even unwillingly, received knowledge of what was required.
This instruction by the law taught exact discernment of how one ought to act in each circumstance. Yet they fulfilled nothing in deed and prided themselves only on this ability to discern what was required" (Oecumenius).