If we say that we have fellowship with Him but walk in darkness we lie and do not the truth.1 John 1:6
John is concerned about a few errors which have cropped up in the early days of the Christian church. John shows us a wonderful example of the truth that fighting wolves is doctrine in action. The first wolf he fights is that of the Nicolaitans. Their namesake, Nicolas, was––according to the early church father Irenaeus1––one of the first seven deacons. They believed they could follow God while indulging in all sorts of fleshly and carnal activities; most notably, they practiced ”spouse-sharing”. In Revelation, we’re told the Lord hates the deeds and doctrine of the Nicolaitans (Rev. 2:6, 15).
So, John in continuing the contrast of light and dark of verse 5 points out that since darkness is not at all in God, it is nonsensical to think that we His children can walk in darkness and still have fellowship with the Light. The Hebrew word for “walk” הלך, carried the idea of one’s manner of life. That sense is continued here in the Greek.
Through the Life manifested in the flesh, Jesus the Christ, we enjoy fellowship with God who is Light. John is teaching that our union with God will result in moral reformation in our “walk” or manner of life. To put it in more systematic theological terms, our justification leads to our sanctification, and to claim that you have fellowship with God while continuing the pursuit of your dark deeds of lust is a lie. We should also note that this lie is presented not doing the truth. An application of this verse is that the truth is something which we should act upon. We should “do the truth” by not walking in darkness, and presuming to also enjoy fellowship with the triune God.