1 John 1:5-7
"This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin."
God is light (phos) — not merely possessing light, but is light. Fellowship with God requires walking in the light. Cleansing by the blood of Christ is continuous (katharizei — present tense) — not a one-time event, but an ongoing process. Walking in the light is not perfection, but transparency and honesty before God.
1 John 1:9
"If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness."
The promise of forgiveness: confession (homologomen — agreeing with God about sin) → forgiveness (aphie) + cleansing (katharize). God is faithful (pistos) — his character guarantees forgiveness; and just (dikaios) — forgiveness is based on righteousness satisfied in Christ.