“That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.”
King James Version (KJV)
1:3 That which we have seen and heard - Of him and from him. Declare we to you - For this end. That ye also may have fellowship with us - May enjoy the same fellowship which we enjoy. And truly our fellowship - Whereby he is in us and we in him. Is with the Father and with the son - Of the Holy Ghost he speaks afterwards.
1Jo 1:3 That ye also may have fellowship. To declare what they had seen and heard was simply to preach the gospel. This knowledge communicated to others brought those who accepted it into a fellowship of knowledge and hope. Those are in fellowship who are partakers of the same faith, hopes, and privileges. With us. All the saints entered into fellowship with the apostles, but the apostles were in fellowship with the Father. This grand fellowship, that of the saints with the Father and the Son, is simply a guarantee that no good thing will be withheld from us; that "all things are ours". There is a fellowship of peace, of concord, of eternal life and glory.