If we should say that we have fellowship with Him, and walk in the darkness, we lie and do not practice the truth, 1 John 1:6.
Since there is no darkness in God, a man who walks in darkness while claiming to have fellowship with God is a liar. Light and darkness have no communion with one another (2 Corinthians 6:14). Such a man who walks in darkness orders his manner of life according to the desires of the sin nature rather than according to God’s desirous will for him.
The truth is the doctrine by which we—those who are saved—have victory over the sin nature. In John 8:32, Jesus declares that His followers will know the truth, and the truth will set them free. Those who did not believe rejected this statement, claiming they had never been slaves to anyone. In John 8:34, Jesus clarifies that He is referring to one who is a slave to sin. The means of victory over the sin nature—the truth—was given to Paul and is recorded for us in Romans 6: Know that we have died with Christ and have been raised to walk in newness of life (Romans 6:4–6); reckon this to be true, for God has stated that we were co-crucified with Christ and raised with Him (Romans 6:11); and yield your members to righteousness (Romans 6:13).