This is the message that we have heard from him and announce to you: "God is light and there is no darkness in him at all."
References for 1 John 1:5
If we claim, "We have fellowship with him," and live in the darkness, we are lying and do not act truthfully.
But if we live in the light in the same way as he is in the light, we have fellowship with each other, and the blood of Jesus, his Son, cleanses us from every sin.
If we claim, "We don't have any sin," we deceive ourselves and the truth is not in us.
But if we confess our sins, he is faithful and just to forgive us our sins and cleanse us from everything we've done wrong.
If we claim, "We have never sinned," we make him a liar and his word is not in us.