Albert Barnes Commentary Matthew 15:17

"Perceive ye not, that whatsoever goeth into the mouth passeth into the belly, and is cast out into the draught?" — Matthew 15:17 (ASV)

Do not you yet understand, etc. The meaning of this may be expressed as follows: The food which is eaten does not affect the mind, and therefore cannot pollute it. The doctrine of the Pharisees, that neglect of washing and of similar observances defiles a person, cannot be true. Those things pertain to the body as much as food does, and they cannot affect the soul.

That must be purified by something else than external washing, and it is polluted by other things than a neglect of mere outward ceremonies. The seat of corruption is within. It is the heart itself; and if one would be made pure, this must be cleansed. If the heart is corrupt, the whole person is corrupt.