Augustine of Hippo : "That ye also," saith he, "may have fellowship with us." And what great matter is it to have fellowship with men? Do not despise it; see what he adds: "and our fellowship may be with God the Father, and Jesus Christ His Son. And these things," saith he, "we write unto you, that your joy may be full." Full joy he means in that fellowship, in that charity, in that unity. — Ten Homilies on 1 John 1 Bede : And we write these things to you so that your joy may be complete. The joy of teachers is made complete when, through preaching, they lead many to the fellowship of the holy Church, and to the fellowship of God the Father and his Son Jesus Christ, through whom the Church is strengthened and grows. Hence also Paul says to those whom he instructed in the faith: Complete my joy by being of the same mind, having the same love, being in full accord and of one mind (Philippians II). — Commentary on the Catholic Epistles Hilary of Arles : The fullness of joy comes when we are in fellowship with the apostles, as well as of the Father, Son and Holy Spirit. — INTRODUCTORY COMMENTARY ON 1 JOHN Oecumenius : "And we write this to you, so that your joy may be complete." However, having seized this, let us be complete with joy as if united with God. Or because while you are participants with us, we have our joy to the fullest; which the joyful sower will establish for the reapers, rejoicing in what they may enjoy from their labors. — Commentary on 1 John