Adam Clarke on Matthew 15:11

Not that which goeth into the mouth defileth - This is an answer to the carping question of the Pharisees, mentioned Mat 15:2, Why do thy disciples eat with unwashed hands? To which our Lord here replies, That what goes into the mouth defiles not the man; i.e. that if, in eating with unwashed hands, any particles of dust, etc., cleaving to the hands, might happen to be taken into the mouth with the food, this did not defile, did not constitute a man a sinner; for it is on this alone the question hinges: thy disciples eat with unwashed hands; therefore they are sinners; for they transgress the tradition of the elders, i.e. the oral law, which they considered equal in authority to the written law; and, indeed, often preferred the former to the latter, so as to make it of none effect, totally to destroy its nature and design, as we have often seen in the preceding notes.

That which cometh out of the mouth - That is, what springs from a corrupt unregenerate heart - a perverse will and impure passions - these defile, i.e. make him a sinner.