“A lying tongue” (Proverbs 6:16-17).
Nothing is more blatant proof of a child of the devil than a lying tongue, because Satan is a liar and the father of lies (John 8:44).
Falsehood in all its forms—slander, blasphemy, gossip, talebearing, lying, “half-truths”, libel, cheating, hypocrisy, fake news, etc.—is the antithesis of Jehovah, the One who cannot even be tempted to sin (James 1:13). This utter holiness is what makes God and His word utterly, absolutely, completely, thoroughly, most assuredly trustworthy.
A tree is known by its fruits (Matthew 7:15, 20). Those things which proceed out of your mouth come from your heart (Matthew 15:18).
Is the fruit of your lips pure or poisoned with falsehood?