The data were examined and the following points were concluded:
1) the Cyprian quotations are not verbatim; 2) the quoted portion (“these three are one”) already exists in v. eight; 3) Cyprian glossed several passages including multiple glosses in John’s first epistle; 4) Cyprian sometimes engaged in mystical interpretation; 5) Cyprian failed to mention it in his most explicit exposition regarding the Trinity; 6) Augustine never cited the Comma despite his reverence for Cyprian; 7) the arguments in favor of authenticity presume a scenario regarding patristic citation that never existed; 8) the silence speaks loudly in light of the concurrent history. The cumulative force of the data suggests that the most probable conclusion is that Cyprian did not quote the Comma but instead found the Trinity in an allegorical interpretation of 1 John 5:8.
One account of its origins suggests that the Comma originated in a Latin homily elaborating on this passage in the Vulgate. The third-century Church father St. Cyprian quoted John 10:30 and added, “Et iterum de Patre et Filio et Spiritu Sancto scriptum est—Et hi tres unum sunt” (De Unitate Ecclesiæ, “On the Unity of the Church”, vi). Translated, Cyprian’s remark reads, “And again it is written of the Father, the Son and the Holy Spirit—and these three are one.” If Cyprian had been aware of the Comma, he would likely have quoted it directly, rather than glossing a verse in a different Johannine book with a sentence which resembles the Comma. Tertullian, in his Against Praxeas (circa 210), also supports a Trinitarian view by quoting John 10:30, even though the Comma would have provided stronger support. Likewise, St. Jerome’s writings of the fourth century give no evidence that he was aware of the Comma’s existence. (The Codex Fuldensis, a copy of the Vulgate made around 546, contains a copy of Jerome’s Prologue to the Canonical Gospels which seems to reference the Comma. However, the Codex’s version of 1 John omits the Comma, which has led many to believe that the Prologue’s reference is spurious.) In the sixth century, St. Fulgentius referred to Cyprian’s remark (in “Responsio contra Arianos”, “Reply against the Arians”). Many figures in the African Church of the period quoted the Comma, but they did so inconsistently; the most notable and prolific writer of the African Church, St. Augustine, is completely silent on the matter.
Metzger, in his Textual Commentary (2nd edition), after commenting on the Greek MS testimony, says this (p. 648):
(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.