Jamieson, Fausset & Brown

Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man--This is expressed even more emphatically in Mark (Mar 7:15-16), and it is there added, "If any man have ears to hear, let him hear." As in Mat 13:9, this so oft-repeated saying seems designed to call attention to the fundamental and universal character of the truth it refers to.