Thou therefore ... - He who is a teacher of others may be expected to be learned himself. They ought to be found to be possessed of superior knowledge; and by this question the apostle impliedly 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 Matthew 23:3, “They say, and do not,” etc.
That preachest - This word means to proclaim in any manner, whether in the synagogue, or in any place of public teaching.
Dost thou 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 mere apparent. We expect a man to set an example of what he means by his public instruction.