Summary: 'Ye have not yet resisted unto blood, striving against sin' (Hebrews 12:4).
THE WAR AGAINST SIN
The scribes and Pharisees brought an accusation to Jesus concerning His disciples (Matthew 15:2). Jesus exposed their hypocrisy with a counter-question (Matthew 15:3), a counter-argument (Matthew 15:4-6), and a counter accusation (Matthew 15:7-9).
This was not about hygiene, as may at first appear, but about certain outward forms and ceremonies. There were many tedious and unnecessary rules and regulations about washing, with meticulous details stipulating which part of the hand should have water poured upon them from which part of the cup at which stage of the procedure. Later the Lord would, in no uncertain terms, warn the crowds about following the lifestyle of the scribes and Pharisees (Matthew 23:4).
Jesus next turned to the crowds (Matthew 15:10-11), and finally to the disciples (Matthew 15:12), with timeless teaching concerning the root of evil (Matthew 15:13). Sin does not consist in that which we put in our mouths (Matthew 15:11). It begins in the heart, and from thence proceeds out of the mouth (Matthew 15:18-20).
Jesus taught elsewhere that adultery begins in the heart when a man looks lustfully on a woman. The sin is entertained in the heart long before it gives expression in the deed, and so Jesus (in a figure) recommends some drastic surgery (Matthew 5:28-30). This is a clarion call to engage the enemy of our souls in the war against sin (Hebrews 12:4).