John Calvin Commentary Romans 2:21

"thou therefore that teachest another, teachest thou not thyself? thou that preachest a man should not steal, dost thou steal?" — Romans 2:21 (ASV)

You, who then teach another, do not teach yourself, and so on. Though the excellencies (encomia — commendations) which he has until now stated respecting the Jews were such as might have justly adorned them, provided the higher ornaments were not lacking; yet as they included qualifications of a neutral kind, which may be possessed even by the ungodly and corrupted by abuse, they are by no means sufficient to constitute true glory.

And so Paul, not satisfied with merely reproving and taunting their arrogance in trusting in these things alone, employs them for the purpose of enhancing their disgraceful conduct. For anyone exposes himself to no ordinary measure of reproach who not only makes useless the gifts of God—gifts which are otherwise valuable and excellent—but by his wickedness also corrupts and contaminates them. And he is a strange counselor who does not consult for his own good and is wise only for the benefit of others. He shows then that the praise they appropriated for themselves turned out to their own disgrace.

You who preach, do not steal, and so on. He seems to have alluded to a passage in Psalm 50:16, where God says to the wicked:

Why do you declare my statutes,
and take my covenant in your mouth?
And you hate reform,
and have cast my words behind you;
when you see a thief, you join him,
and your portion is with adulterers.

And as this reproof was suitable to the Jews in former times, who, relying on the mere knowledge of the law, lived in no way better than if they had no law, so we must take heed, lest it should be turned against us today. Indeed, it may be well applied to many who, boasting of some extraordinary knowledge of the gospel, abandon themselves to every kind of uncleanness, as though the gospel were not a rule of life.

So that we may not then so heedlessly trifle with the Lord, let us remember what sort of judgment impends over such prattlers, (logodœdalis — word-artificers), who make a show of God’s word by mere talkativeness.