(2) The passage is quoted in none of the Greek Fathers, who, had they known it, would most certainly have employed it in the Trinitarian controversies (Sabellian and Arian). Its first appearance in Greek is in a Greek version of the (Latin) Acts of the Lateran Council in 1215.
(3) The passage is absent from the manuscripts of all ancient versions (Syriac, Coptic, Armenian, Ethiopic, Arabic, Slavonic), except the Latin; and it is not found (a) in the Old Latin in its early form (Tertullian Cyprian Augustine), or in the Vulgate (b) as issued by Jerome ... or (c) as revised by Alcuin...
The earliest instance of the passage being quoted as a part of the actual text of the Epistle [italics added] is in a fourth century Latin treatise entitled Liber Apologeticus (chap. 4), attributed either to the Spanish heretic Priscillian (died about 385) or to his follower Bishop Instantius. Apparently the gloss arose when the original passage was understood to symbolize the Trinity (through the mention of three witnesses: the Spirit, the water, and the blood), an interpretation that may have been written first as a marginal note that afterwards found its way into the text.
Thus, a careful distinction needs to be made between the actual text used by Cyprian and his theological interpretations. As Metzger says, the Old Latin text used by Cyprian shows no evidence of this gloss. On the other side of the ledger, however, Cyprian does show evidence of putting a theological spin on 1 John 5:7. In his De catholicae ecclesiae unitate 6, he says, “The Lord says, ‘I and the Father are one’; and again it is written of the Father, and of the Son, and of the Holy Spirit, ‘And these three are one.’” What is evident is that Cyprian’s interpretation of 1 John 5:7 is that the three witnesses refer to the Trinity. Apparently, he was prompted to read such into the text here because of the heresies he was fighting (a common indulgence of the early patristic writers). Since John 10:30 triggered the ‘oneness’ motif, and involved Father and Son, it was a natural step for Cyprian to find another text that spoke of the Spirit, using the same kind of language. It is quite significant, however, that (a) he does not quote ‘of the Father, and of the Son, and of the Spirit’ as part of the text; this is obviously his interpretation of ‘the Spirit, the water, and the blood.’ (b) Further, since the statement about the Trinity in the Comma is quite clear (“the Father, the Word, and the Holy Spirit”), and since Cyprian does not quote that part of the text, this in the least does not afford proof that he knew of such wording. One would expect him to quote the exact wording of the text, if its meaning were plain. That he does not do so indicates that a Trinitarian interpretation was superimposed on the text by Cyprian, but he did not change the words. It is interesting that Michael Maynard, a TR advocate who has written a fairly thick volume defending the Comma (A History of the Debate over 1 John 5:7-8 [Tempe, AZ: Comma Publications, 1995] 38), not only quotes from this passage but also speaks of the significance of Cyprian’s comment, quoting Kenyon’s Textual Criticism of the New Testament(London: Macmillan, 1912), 212: “Cyprian is regarded as one ‘who quotes copiously and textually’.” The quotation from Kenyon is true, but quite beside the point, for Cyprian’s quoted material from 1 John 5 is only the clause, “and these three are one”—the wording of which occurs in the Greek text, regardless of how one views the Comma.
Thus, that Cyprian interpreted 1 John 5:7-8 to refer to the Trinity is likely; but that he saw the Trinitarian formula in the text is rather unlikely. Further, one of the great historical problems of regarding the Comma as authentic is how it escaped all Greek witnesses for a millennium and a half. That it at first shows up in Latin, starting with Priscillian in c. 380 (as even the hard evidence provided by Maynard shows), explains why it is not found in the early or even the majority of Greek witnesses. All the historical data point in one of two directions:
(1) This reading was a gloss added by Latin patristic writers whose interpretive zeal caused them to insert these words into Holy Writ; or
(2) this interpretation was a gloss, written in the margins of some Latin MSS, probably sometime between 250 and 350, that got incorporated into the text by a scribe who was not sure whether it was a comment on scripture or scripture itself (a phenomenon that was not uncommon with scribes).