John's concrete experience with Jesus forms the bedrock of the authority of this letter. He has first-hand experience and wants us to know that he is passing on to us what he got from Jesus.
Life! Jesus is life. His words bring life. Christianity is not arguments about doctrine, its about life! That life comes in fellowship with God. A relationship with God, unbroken by sin, is what life is all about. Prospectively, walk in the light and you will have fellowship with him.
If you are trapped in darkness, confess your sin and he will forgive and cleanse you. Don't deceive yourself; don't hide or pretend that some how you don't have sin.