1 John 1:3 That which we have seen and heard declare we unto you, that ye may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.
John and the Apostles were eyewitnesses of the glory of God revealed in Jesus Christ, and they are proclaiming it to us for this purpose: fellowship. God promises fellowship, an intimate personal relationship. This is the purpose and the promise of the Word of God. Fellowship normally implies a loving relationship between more than two persons. The three persons of the Trinity share their love and life with each other so perfectly that they are One. The Triune God desires, and promises, that same kind of unified, loving relationship with us.
John was present when Jesus revealed His Glory (the perfect love of God) on the mounts of Transfiguration, Calvary, Tomb, and Ascension. The Apostles were inspired to write down their testimony for the purpose of our fellowship with God.
”These things are written that you may believe, and that believing you may have life in his name.” The Spirit uses the written and oral proclamation of the Word to bring us to faith and eternal life.
Once we have faith in Jesus we enter into a personal relationship with the Almighty, Eternal God and with all other believers in an unbroken fellowship. We enjoy the mutual conversation and consolation of the brothers and sisters in Christ (Jesus with skin on); and we enjoy a continual back and forth conversation with Jesus all the time. This continual fellowship is built up and reinforced by particular times of hearing the Gospel and receiving the Sacrament and responding with prayer and praise and doing good to others.