How will you instruct the elect of the Lord, if you yourselves have not instruction? Instruct each other therefore, and be at peace among yourselves, that
Hence his invective against the transgressors of the law, who teach that men should not steal, and yet practise theft themselves. (This invective he utters) in perfect homage to the law of God, not as if he meant to ten sure the Creator Himself with having commanded a fraud to be practised against the Egyptians to get their gold and silver at the very time when He was forbidding men to steal, -adopting such methods as they are apt (shamelessly) to charge upon Him in other particulars also.
This means: “You who complain about the Gentiles because they are without the law and God are accusing yourself, because you do not believe in the Christ promised by the law but find this belief in those you are complaining about.” The Jew does what he preaches should not be done. For by denying the Christ promised to us in the law, he removes faith by false interpretation and thus does what he preaches against.
But there he frames his speech with more of sharpness, here with more of gentleness. For he does not say, However on this score you deserve greater punishment, because though entrusted with so great things you have not made a good use of any of them, but he carries his discourse on by way of question, turning them on themselves (ἐ ντρέπων]), and saying, "Thou that teachest another, do you not teach yourself?" And here I would have you look at the discretion of Paul in another case. For he sets down such advantages of the Jews, as came not of their own earnestness, but by a gift from above, and he shows not only that they are worthless to them if neglectful, but that they even bring with them increase of punishment. For neither is the being called a Jew any well doing of theirs, nor yet is the receiving of the Law, nor the other things he has just enumerated, but of the grace from above. And towards the beginning he had said, that the hearing of the Law is valueless unless the doing be thereto added ("for not the hearers of the Law," he says, "are just before God,") but now he shows further still, that not only the hearing, but, what is more than the hearing, the teaching of the Law itself will not be able to screen the teacher, unless he do what he says; and not only will it not screen him, but will even punish him the more. And he has used his expressions well too, since he does not say, You have received the Law, but "You rest in the Law." For the Jew was not wearied with going about to seek what was to be done, but had on easy terms the Law pointing the way leading to virtue. For if even the Gentiles have natural reason (and it is on this ground that these are better than they, in that they do the Law without hearing), yet still the others had greater facility. But if you say, I am not only a hearer, but even a teacher, this very thing is an aggravation of your punishment. For because they prided themselves upon this, from this above all he shows them to be ridiculous. But when he says, "a guide of the blind, an instructor of the foolish, a teacher of babes," he is speaking their own pompous language. For they treated proselytes extremely ill, and these were the names they called them by. And this is why he dwells at large upon what were supposed to be their praises, well knowing that what was said gave ground for greater accusation; "Which hast the form of knowledge and of the truth in the Law." As if any one who had a picture of the king, were to draw nothing after it, and they that were not entrusted with it were to imitate it exactly even without the original. And then after mentioning the advantages they had from God, he tells them of their failings, bringing forward what the prophets accused them of. "You therefore who teaches another, do you not teach yourself? You that preaches that a man should not steal, do you steal? You who says a man should not commit adultery, do you commit adultery? You that abhors idols, do you commit sacrilege?" For it was strictly forbidden them to touch any of the treasures upon the idols (so Field from the manuscripts: Vulg. "in the idol temples") by reason of the defilement. But the tyranny of avarice, he says, has persuaded you (4 manuscripts and mar. "us") to trample this Law also under foot.
Sometimes Paul speaks more harshly and sometimes more softly. Here he does not attack them directly … but carries on his discourse by way of questions, getting them to give the answers themselves.
Paul says to the Jew: If you rely on the law, why do you not obey it? If you glory in God, why do you dishonor him? If you know his will, why do you not do it? If you approve what is excellent, why do you go after what is harmful? Why do you not look for the right way, if you are a guide for the blind? Surely if you saw it you would walk in it! If you are a light for others, why do you not cast off the works of darkness? As an instructor of the foolish, why have you abandoned the fear of the Lord, which is the beginning of wisdom? As a teacher of children, why are you a child in understanding? If you have the standard of knowledge and truth in the law, why do you not follow it yourself, nor by your evil example allow others to follow it? Why does your life not match your teaching, and why does your behavior make a mockery of your faith? Because you have not kept the law it will happen that not only will the law do you no good, it will condemn you for the greater crime of holding it in contempt.
Paul shows here and in the next two verses that the Jews had learned little from the law even if they gloried in the letter of it. When they tried to teach others, their deeds contradicted their words, and their pride in the law was pointless.
Poemen also said, ‘Teach your heart to follow what your tongue is saying to others.’ He also said, ‘Men try to appear excellent in preaching but they are less excellent in practising what they preach.’
[AD 160] Shepherd of Hermas on Romans 2:21