He who says he is in the light and hates his brother, is in the darkness even until now.
He ... is in darkness even until now ...  It is a mistake to understand John as merely refuting the erroneous teachings of Gnostics.  While it may be true enough that Gnostics might have seduced many Christians of that era into receiving a lifestyle of "loveless arrogance,"[26] the great teachings of the apostle were not merely a reaction to such things.  He was not merely reacting; he was proclaiming the tremendous truths already revealed by Jesus nearly a whole generation previously.  In the sermon on the mount, Jesus said: "If thine eye be evil, thy whole body shall be full of darkness.  If therefore the light that is in thee be darkness, how great is that darkness"  (Matthew 6:23).  Thus, the metaphor of light and darkness goes back to Christ himself.  Paul likewise received and used the same metaphor, his message in  Ephesians 4:17-18 reading thus: "For they live blindfolded in a world of illusion, and are cut off from the life of God through ignorance and insensitiveness."[27] He also wrote, "Cast off the works of darkness"  (Romans 13:12), "We are not of the night, nor of darkness"  (1 Thessalonians 5:5), "What communion hath light with darkness?"   (2 Corinthians 6:14).