5:1 - Whosoever believes that Jesus is the Christ is born of God: and every one that loves him that brings forth loves him also that is begotten of him.

Irenaeus Against Heresies Book III
" Wherefore he again exclaims in his Epistle, "Every one that believeth that Jesus is the Christ, has been born of God; "[298]

5:3 - For this is the love of God, that we keep his commandments: and his commandments are not grievous.

Clement of Alexandria The Instructor Book III
For the love meant is the love of God. "And this is the love of God," says John, "that we keep His commandments; "[157]
Clement of Alexandria Stromata Book IV
"This is the love of God, that we keep His commandments."[145]

5:6 - This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that bears witness, because the Spirit is truth.

Irenaeus Against Heresies Book IV
, and calls them into his own kingdom? And why is his goodness, which does not save all [thus], defective? Also, why does he, indeed, seem to be good as respects men, but most unjust with regard to him who made men, inasmuch as he deprives him of his possessions? Moreover, how could the Lord, with any justice, if He belonged to another father, have acknowledged the bread to be His body, while He took it from that creation to which we belong, and affirmed the mixed cup to be His blood?[470]
Tertullian On Baptism
when He had been baptized already. For He had come "by means of water and blood,"[160]
A Treatise on Re-Baptism by an Anonymous Writer
For John says of our Lord in his epistle, teaching us: "This is He who came by water and blood, Jesus Christ; not by water only, but by water and blood: and it is the Spirit that beareth witness, because the Spirit is truth. For three bear witness, the Spirit, and the water, and the blood: and these three are one; "[48]

5:7 - For there are three that bear record in heaven, the Father, the Word, and the Holy Spirit: and these three are one.

Cyprian Treatise I On the Unity of the Church
and again it is written of the Father, and of the Son, and of the Holy Spirit, "And these three are one."[22]

5:8 - And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.

A Treatise on Re-Baptism by an Anonymous Writer
Moreover, I think also that we have not unsuitably set in order the teaching of the Apostle John, who says that "three bear witness, the Spirit, and the water, and the blood; and these three are one."[61]
Origen Commentary on John Book VI
"I have a baptism to be baptized with, and how am I straitened till it be accomplished? "And it agrees with this that the disciple John speaks in his Epistle[127]

5:11 - And this is the record, that God has given to us eternal life, and this life is in his Son.

Shepherd of Hermas Similitude Ninth
Accordingly, those also who fell asleep received the seal of the Son of God. For," he continued, "before a man bears the name of the Son of God[27]

5:12 - He that has the Son has life; and he that has not the Son of God has not life.

Tertullian Against Praxeas
We believe not the testimony of God in which He testifies to us of His Son. "He that hath not the Son, hath not life."[437]

5:16 - If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.

Shepherd of Hermas Similitude Sixth
These have been perverted from the truth: among them there is the hope of repentance, by which it is possible to live. Corruption, then, has a hope of a kind of renewal,[7]
Clement of Alexandria Stromata Book II
For"there is a sin unto death: I do not say that one is to pray for it. All unrighteousness is sin; and there is a sin not unto death."[152]
Tertullian On Modesty
For (in making these assertions) he was looking forward to the final clause of his letter, and for that (final clause) he was laying his preliminary bases; intending to say, in the end, more manifestly: "If any knoweth his brother to be sinning a sin not unto death, he shall make request, and the Lord shall give life to him who sinneth not unto death. For there is a sin unto death: not concerning that do I say that one should make request."[252]
Hippolytus Refutation of All Heresies Book IX
unto death,[54]
Cyprian Treatise XII Three Books of Testimonies Against the Jews
Of this same thing in the first book of Kings: "If a man sin by offending against a man, they shall pray the Lord for him; but if a man sin against God, who shall pray for him? "[557]
Origen Commentary on Matthew Book XIII
at such words are spoken about every sin, whether the sin be murder, or poisoning, or paederasty, or anything of that sort, would give occasion of injury to the exceeding goodness of Christ, so, on the contrary, he who distinguishes between the brother and him who is called the brother, might teach that, in the case of the least of the sins of men, he who has not repented after the telling of the fault is to be reckoned as a Gentile and a publican, for sins which are "not unto death,"[193]

5:19 - And we know that we are of God, and the whole world lies in wickedness.

Origen de Principiis Book I
, of an earthly habitation: for he exercised power over those who were obedient to his wickedness, since "the whole of this world"-for I term this place of earth, world-"lieth in the wicked one,"[103]
Origen de Principiis Book II
That earth of ours, with its inhabitants, is also termed the world, as when Scripture says, "The whole world lieth in wickedness."[22]
Dionysius The Gospel According to Luke
to remain altogether without experience of ill. For, as one says, the whole world lieth in wickedness; "[28]
Dionysius An Exposition of Luke XXII. 46
For in the most general application it holds good, that it does not appear to be possible for any man to remain altogether without experience of ill: for, as one says, "The whole world lieth in wickedness; "[3]
Archelaus Acts of the Disputation with the Heresiarch Manes
in the portion of the wicked one, as John says, that "the whole world lieth in wickedness,"[121]
Methodius From the Discourse on the Resurrection
" As then, when the days of our present life shall fail, those good deeds of beneficence to which we have attained in this unrighteous life, and in this "world" which "lieth in wickedness,"[94]

5:21 - Little children, keep yourselves from idols. Amen.

Tertullian De Corona
Even an earthly serpent sucks in men at some distance with its breath. Going still further, John says, "My little children, keep yourselves from idols,"[37]