1 John 1:7

Because there are Three which bear witness in Heaven, &c. S. John here more fully explains and confirms the testimony already adduced of the water, the blood and the spirit concerning Christ. The particle because is partly confirmatory of what He had said in the 5th ver. that Jesus is the Son of God, partly of what he said in the 6th, that the Spirit bears witness that Jesus is true God. For this is here confirmed because the Holy Spirit is one of the three witnesses who in heaven bear testimony to Christ.

S. Jerome (Pref. in Epist. Canon.) observes that this verse had been erased by unbelievers, i.e. the Arians, from some Greek copies. Therefore it is not found in the Syriac, Clement of Alexandria, Bede, Œcumenius, and some others. It is, however, the constant reading of the Latin Bibles, and the more correct Greek MSS. and of many of the ancients, SS. Athanasius, Augustine, Jerome, Cyprian, the Lateran Council, at which Greeks were present. Therefore it is certain that these words are to be taken as canonical Scripture.

The meaning then is this All the Three Persons of the Sacred Trinity in heaven and from heaven bear testimony to the angels, but especially to men (for to men S. John chiefly refers) concerning Christ, that He is the true Messiah and the Son of God. This the Father did at His Baptism and Transfiguration. Again, when He answered Christ by thunder out of heaven, "I have both glorified, and will glorify Thee again " (John 12:18). Similarly also the Holy Ghost bore witness when He descended upon Christ in the form of a dove, and poured out Himself upon the Apostles and other Christians at Pentecost. And this was the result of Christ's prediction, promise, and mission. Wherefore the same Holy Spirit by the mouths of the Apostles preached little else save Christ. The Son also very often declared, taught, and proved convincingly by His miracles that He was the Messias and the Son of God, as is plain from the whole Gospel of S. John. Therefore heaven and earth mutually agree, yea the whole universe appears at one, in bearing this witness to Christ.

And these Three are One as in nature and Divine Essence, so likewise in intelligence, voice, and testimony, concerning Christ. For all these things in the Holy Trinity are one and the same. There is another reading of these words in the Greek, signifying These Three are into One (in unum), but the Latin and other Greek copies have These Three are One (Hi tres unum suns), signifying the oneness of Substance of the Holy Trinity, that the Three Persons have one and the same undivided Godhead.