4 And these things write we unto you, that your joy may be full.
[AD 215] Clement of Alexandria on 1 John 1:1-5
Chap. i. 1. "That which was from the beginning; which we have seen with our eyes; which we have heard."

Following the Gospel according to John, and in accordance with it, this Epistle also contains the spiritual principle.

What therefore he says, "from the beginning," the Presbyter explained to this effect, that the beginning of generation is not separated from the beginning of the Creator. For when he says, "That which was from the beginning," he touches upon the generation without beginning of the Son, who is co-existent with the Father. There was; then, a Word importing an unbeginning eternity; as also the Word itself, that is, the Son of God, who being, by equality of substance, one with the Father, is eternal and uncreate. That He was always the Word, is signified by saying, "In the beginning was the Word." But by the expression, "we have seen with our eyes," he signifies the Lord's presence in the flesh, "and our hands have handled," he says, "of the Word of life." He means not only His flesh, but the virtues of the Son, like the sunbeam which penetrates to the lowest places — this sunbeam coming in the flesh became palpable to the disciples. It is accordingly related in traditions, that John, touching the outward body itself, sent his hand deep down into it, and that the solidity of the flesh offered no obstacle, but gave way to the hand of the disciple.

"And our hands have handled of the Word of life;" that is, He who came in the flesh became capable of being touched. As also,

Ver. 2. "The life was manifested." For in the Gospel he thus speaks: "And what was made, in Him was life, and the life was the light of men." [John 1:3-4]

"And we show unto you that eternal life, which was with the Father, and was manifested unto you."

He signifies by the appellation of Father, that the Son also existed always, without beginning.

Ver. 5. "For God," he says, "is light."

He does not express the divine essence, but wishing to declare the majesty of God, he has applied to the Divinity what is best and most excellent in the view of men. Thus also Paul, when he speaks of "light inaccessible." [1 Timothy 6:16] But John himself also in this same Epistle says, "God is love:" [1 John 4:16] pointing out the excellences of God, that He is kind and merciful; and because He is light, makes men righteous, according to the advancement of the soul, through charity. God, then, who is ineffable in respect of His substance, is light.

"And in Him is no darkness at all,"— that is, no passion, no keeping up of evil respecting any one, [He] destroys no one, but gives salvation to all. Light moreover signifies, either the precepts of the Law, or faith, or doctrine. Darkness is the opposite of these things. Not as if there were another way; since there is only one way according to the divine precepts. For the work of God is unity. Duality and all else that exists, except unity, arises from perversity of life.

[AD 430] Augustine of Hippo on 1 John 1:4
"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.

[AD 449] Hilary of Arles on 1 John 1:4
The fullness of joy comes when we are in fellowship with the apostles, as well as of the Father, Son and Holy Spirit.

[AD 550] Oecumenius on 1 John 1:2-4
And we declare to you eternal life, which was with the Father and was revealed to us. What we have seen and heard, we also declare to you, so that you may have fellowship with us, and our fellowship is with the Father and His Son Jesus Christ: And we write this to you, so that your joy may be complete.
"And we declare to you the life." Therefore, what we have seen, we also declare to you, John says. What is that? That he who is eternal life has been revealed to us, and we have become witnesses of him both before his crucifixion and after his resurrection. For he was indeed nailed to the cross in the flesh, and in that same flesh he rose again. And what profit do we bring you from this announcement? Namely, that just as we have taken you as partners [κοινωνοὺς] through the words of what we have seen and heard, so you may also be partners of the Father and of his Son Jesus Christ.
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.

[AD 735] Bede on 1 John 1:4
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).

[AD 1107] Theophylact of Ohrid on 1 John 1:4
For when we are in communion with you, we experience the greatest pleasure, similar to that which a rejoicing sower provides to the reapers at the distribution of wages, when they too rejoice that others enjoy their labors.