“For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.”
Matthew 15 begins with the Pharisees being offended that Christ’s disciples didn’t follow their traditions of ritual hand washing. This led Christ to discuss not only their traditions, but the real source of defilement.
Christ told everyone, “Not what goes into the mouth defiles a man; but what comes out of the mouth, this defiles a man” (verse 11). Peter asked Him to explain, and Christ showed what is truly important. If some dirt gets on our food, that’s just a physical thing that will be eliminated by the digestive system. But what comes out of our hearts reflects our spiritual condition. Eating dirt is nothing compared to breaking God’s laws.
For more about how our thoughts and temptations lead to sin, see “Temptation.”