John identifies the ultimate liar as one who denies that Jesus is the Christ. Commentators explain this goes beyond simply denying Jesus's existence. In John's time, it referred to Gnostic teachers who separated the human Jesus from the divine Christ, diminishing His incarnation and atoning death. Today, any teaching that undermines Jesus's true identity as both fully God and fully man, the promised Messiah, falls into this category of fundamental falsehood.