William Burkitt on Matthew 15:10

Our blessed Saviour leaving the Pharisees with some dislike, applies himself to the multitude, and shews them the true spring and original fountain of all spiritual pollution and uncleanness; namely, the filthiness and impurity of man's heart and nature, which boiling in the heart, the scum runs out at the mouth: thereby informing the multitude, that not that which is eaten, but that which is spoken, defiles a man: not the meat eaten with the mouth, but the wickedness of the heart vented by the mouth, pollutes a person in God's account.