1That which was from the beginning, that which we have heard, that which we have seen with our eyes, that which we saw, and our hands touched, concerning the Word of life 2(and the life was revealed, and we have seen, and testify, and declare to you the life, the eternal life, which was with the Father, and was revealed to us); 3that which we have seen and heard we declare to you, that you also may have fellowship with us. Yes, and our fellowship is with the Father and with his Son, Jesus Christ. 4And we write these things to you, that our joy may be fulfilled.
5This is the message which we have heard from him and announce to you, that God is light, and in him is no darkness at all. 6If we say that we have fellowship with him and walk in the darkness, we lie and don’t tell the truth. 7But if we walk in the light as he is in the light, we have fellowship with one another, and the blood of Jesus Christ his Son, cleanses us from all sin. 8If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9If we confess our sins, he is faithful and righteous to forgive us the sins and to cleanse us from all unrighteousness. 10If we say that we haven’t sinned, we make him a liar, and his word is not in us.
Fellowship (Gk. κοινωνία, koinonia). (Acts 2:42; 1 Cor 1:9; 2 Cor. 6:14; Phil 1:5; Heb 13:16; 1 John 1:3, 6, 7) Strong’s 2842
This word is used nineteen times in the NT, but not until Acts 2:42. Thirteen of these uses are in Paul. This word can describe a close association involving mutual interest (partnership), an attitude of good will manifest in a close relationship, a sign of fellowship (gift) or participation/sharing. This word is used often to describe the relationship and sharing enjoyed by Christians. In this passage, the meaning has to do with association based on mutual interest. Light and dark are opposites, just as the purpose and drive of a believer is the opposite of an unbeliever.