Deceitful means insincere, hypocritical, underhanded, false, dishonest, treacherous, sneaking, double-dealing, tricky, cunning, and crafty. Such a person is altogether untrustworthy. As Jeremiah 17:9 says, our heart is desperately sick or weak, implying it knows better but deceives anyway. Who can fathom its corruption, manifested in the incessant transgression of this commandment?
Human nature is a reflection of the spirit of the prince of the power of the air, whom Jesus identified as the father or generator of lies (John 8:44). Satan had so deceived himself, he thought he could overcome his Creator! Proverbs 11:9 says, "The hypocrite with his mouth destroys his neighbor, but through knowledge the righteous will be delivered." Satan is a destroyer who passes this carnal attribute along to those who will follow him. Unless the hypocrite repents, he destroys himself too. This is also the lesson of Proverbs 26:26-28. God will deliver the just person, however, because he yields to truth.