Fellowship with Christ, Walking in the Light

John is addressing fellowship with God and Jesus. John makes the argument, because he and his team have fellowship with God and Jesus, if you fellowship with John, then you have fellowship with God and Jesus. This is predicated on the fact that you are walking in the light. John describes God as the light, so when we are washed of our sins, we are also walking in the light.
John introduces his goal of fellowship in verses 3-4. God is the light (v. 5), then John creates the argument using dualism techniques and conditional clauses (v. 6-7, 8-9). He then finishes up with the final conditional clause by making the statement: If we say we that have not sinned…” (v. 10). The good news is that God sent Jesus to wash us clean and redeem us from our sins and to provide a path for forgiveness from His Father (v. 2:1-2)