Albert Barnes 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 therefore, etc. One who is a teacher of others may be expected to be learned himself. They ought to possess superior knowledge; and by this question the apostle implicitly reproves them for their ignorance. The form of a question is chosen because it conveys the truth with greater force. He puts the question as if it were undeniable that they were grossly ignorant. (Compare to Matthew 23:3, They say, and do not, etc.).

That preach. This word means to proclaim in any manner, whether in the synagogue, or in any place of public teaching.

Do you steal? It cannot be proved, perhaps, that the Jews were extensively guilty of this crime. It is introduced partly, no doubt, to make the inconsistency of their conduct more apparent. We expect a man to set an example of what he means by his public instruction.