This is an interesting parallel to Peter's denial (Luke 22:54-62). Peter says three times "I don't know him". Peter had the truth in him; he knew everything that Jesus had taught during those three years. He received his education in the faith from God Himself. But Peter did not speak the truth.
What we're warned against today is the opposite - speaking the truth but not meaning it. Not living as if what we say is true. Preaching with our lips but not our hearts or our hands.
Some Christians ask the question "have you found Jesus?" That is not the real challenge. Jesus is easy to find. He has made Himself incredibly available. The real question is whether Jesus has found you. Has He found a way into your heart and into your daily life? Or have you found Him and passed by?