For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. (Matthew 15:19, NKJV)
All sin proceeds from the heart. So, Jesus teaches us to cleanse our hearts to be pure (Matthew 15:1-9, 18). It is not coincidental the Lord begins this catalog of sins come from the heart with “evil thoughts.” Evil thoughts are the reasonings and contemplations that lead to sinful conduct. The heart considers evil before committing evil. It is this contemplation of evil in the heart that is condemned here. We are not to entertain the thought of evil, much less do the evil we contemplate. This is not the way the world thinks about sin. Many people say, “No harm, no foul,” as if only doing the evil is wrong. But, the Son of God said evil thoughts harm us and our standing with God. For instance, Jesus said to look at a woman to lust after her is to already commit adultery with her in one’s heart (Matthew 5:28). It is the pure in heart who shall see God (Matthew 5:8). So, purify the thoughts of your heart of every lust, covetousness, deceit, bitterness, vengeance, partiality and all other evil thinking that leads to sinful conduct (James 4:8). We must singularly devote our thoughts to the things of God.