VERSE 3: FELLOWSHIP
what we have seen and heard we also declare to you, so that you may also have fellowship with us; and indeed our fellowship is with the Father and with his Son Jesus Christ.
John lays out the chief aims of his endeavor to unfold the eternal life that has been manifest to us from the Father: to bring believers into the fellowship of the saints and their fellowship with the Father and his Son Jesus Christ.
One of the hallmarks of apostolic doctrine is its concern for the unity of the body (See Ephesians 4:3). When our doctrine shows no concern for the unity of the body, and considers frivolous division as a matter of little consequence, whatever we are preaching, it is not the doctrine of Christ. Undoubtedly, the Light of Truth will divide those who love darkness from those who love the light. But God hates one who divides brothers (See Proverbs 6:19)!
John writes that our fellowship is in the unity of our faith both with the purchased possession of Christ and with God and his Son Jesus Christ.
Andreas (c. seventh century), a monk who collected commentary from earlier writers to form a catena on various biblical books, wrote the following concerning this verse:
What did they proclaim, but that eternal life has appeared to us and that we have become witnesses of it? What you gain from this proclamation is the right to share this experience with us. For the one who is in fellowship with us has fellowship with the Father and with his Son, Jesus Christ, and since you share in the same fellowship, we shall all have joy together, in that we have been united to God.
What is meant by “fellowship”?
The word “κοινωνίαν” is difficult to capture in English and has been variously translated as “fellowship,” “communion,” “participation,” “share a common life,” and “partnership”; its root meaning is “common” or “shared” as opposed to “one’s own.” Hellenistic literature uses it to describe partners in business, joint owners of a piece of property, or shareholders in a common enterprise. In the NT it refers to Christians who share a common faith (Philem 6), who share possessions (Acts 2:44; 4:30), or who are partners in the gospel (Philippians 1:5). κοινωνίαν, with its derivatives, occurs over sixty times in the NT in reference to the supernatural life that Christians share. This supernatural life is disclosed in the incarnate Christ. It is the eternal life that comes from the Father and becomes the life shared individually and corporately by the company of believers. It is what causes the oneness of faith.
In other words, we share in the eternal life that has been manifested to us from the Father in the beginning through faith in the gospel of Christ that John is here declaring to us.
The order of fellowship is significant: first, we have fellowship with the assembly of saints who share in the manifest eternal life, then we have fellowship with the Father and his Son Jesus Christ. This reflects the discourse in chapter four, where John writes, “If anyone says, ‘I love God,’ and yet hates his brother or sister, he is a liar. For the person who does not love his brother or sister whom he has seen cannot love God whom he has not seen.” Even though John has seen Jesus, and therefore has seen the Father (See John 14:9), he is writing to those who have not seen Jesus. The Church is the body of Christ and it is in fellowship with the assembly of believers that we have fellowship with God!
Our Western independence has made it difficult to grasp the importance of community fellowship. But Christ established a community of believers so that “From him the whole body, fitted and knit together by every supporting ligament, promotes the growth of the body for building up itself in love by the proper working of each individual part”(Eph. 4:16).
In other words, we do not grow up in the love of Christ alone, but in community with the whole body of Christ. Christian fellowship means sharing eternal life in Christ through the Holy Spirit to the everlasting glory of God!
Biblical Reasons 