“It is not what enters into the mouth which defiles the man, but what proceeds out of the mouth, this defiles the man.”
Genuine religious defilement in the eyes of God is not caused by what men eat, but by what is inside a man and comes out in what he says. As He has previously warned, ‘For every idle word that a man shall speak, he will give account of it in the Day of Judgment' (Matthew 12:36). It is such words that reveal what is truly in a man's heart. If the question is, ‘How are we to tell what a man is really like?', the reply is, ‘Listen, not to his prepared words, but to his idle words', his words spoken when he is off guard. Then we will know what is truly in his heart.
So Jesus is bringing out the lesson that the most defiling thing about a man is his sinfulness. It is found in what he thinks, and reasons and wills. It is not found in what has been made unclean by touch. By this Jesus was seeking to turn people from an obsession with religious ritual, to genuine godliness of living. His point is that God was most pleased when His people lived righteously and compassionately, as the prophets had constantly said. (See e.g. Isaiah 1:11-20; Micah 6:8).