The Johannine Comma (1 John 5:7) reads:
For there are three who bear witness in heaven: the Father, the word, and the Holy Spirit; and these three are one
Inauthenticity
Bruce M. Metzger writes:
That these words are spurious and have no right to stand in the New Testament is certain in the light of the following considerations.
(A) External Evidence:
(1) The passage is absent from every known Greek manuscript except four, and these contain the passage in what appears to be a translation from a late recension of the Latin Vulgate. These four manuscripts are ms. 61, a sixteenth century manuscript formerly at Oxford, now at Dublin; ms. 88, a twelfth century manuscript at Naples, which has the passage written in the margin by a modern hand; ms. 629, a fourteenth or fifteenth century manuscript in the Vatican; and ms. 635, an eleventh century manuscript which has the passage written in the margin by a seventeenth century hand.

(2) The passage is quoted by 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 (codex Fuldensis [copied a.p. 541-46] and codex Amiatinus [copied before a.p. 716]) or (c) as revised by Alcuin (first hand of codex Vercellensis {ninth century]).
(B) Internal Probabilities:
(1) As regards transcriptional probability, if the passage were original, no good reason can be found to account for its omission, either accidentally or intentionally, by copyists of hundreds of Greek manuscripts, and by translators of ancient versions.

(2) As regards intrinsic probability, the passage makes an awkward break in the sense.For the story of how the spurious words came to be included in the Textus Receptus, see any critical commentary on 1 John, or Metzger, The Text of the New Testament, pp. 101 f.; cf. also Ezra Abbot, “I. John v. 7 and Luther’s German Bible,” in The Authorship of the Fourth Gospel and Other Critical Essays (Boston, 1888), pp. 458-463.”

1. Neil R. Lightfoot writes:
2. The textual evidence is against 1 John 5:7. Of all the Greek manuscripts, only two contain it. These two manuscripts are of very late dates, one from the fourteenth or fifteenth century and the other from the sixteenth century. Two other manuscripts have this verse written in the margin. All four manuscripts show that this verse was apparently translated from a late form of the Latin Vulgate.

Did Early Church Fathers quote the Johannine Comma?
The most common defense of 1 John 5:7 is appealing to church fathers who supposdely quote the verse in their writings, specficially they refer Cyprian of Carthage.
Did Cyprian of Carthage quote 1 John 5:7?
Bill Brown, a New Testament scholar has written a great paper titled “Did Cyprian Quote the Comma Johanneum?” in which he refutes the idea Cyprian quotes the verse. He points out 5 major issues with this.

As he writes:
First, it is not a verbatim quotation. A verbatim quotation would reference the “Father, Word, and Holy Spirit,” a distinctive phrase that occurs nowhere else in Scripture. Cyprian quotes “Son,” (filio) not “Word” (verbum). Although verbatim quoting is not always determinative, it plays an important role in evaluating patristic citations. In the immediate context of the quotation (et tres unum sunt), Cyprian references many Scriptures, including Gen 7:20; Matt 12:20; John 10:30; 2 Cor 11:2; Eph 5:23; and1 Pet 3:20. It is therefore possible that he was referencing the language of Matt 28:19 combined with 1 John 5:8. The quotation does not reference anything distinctly found in the Comma.

Secondly, the phrase “et tres unum sun” occurs regardless of whether the Comma is included. If Cyprian’s tendency was to quote Scripture verbatim, it is difficult to believe that he would have said “Son” (Filio) if he read “Word” (Verbum) in his text. This is a double-edged sword: Comma advocates insist Cyprian quoted textually but overlook the fact he never quotes “Father, Word, and Holy Spirit.” This quotation suggests that at least one of those two assumptions is incorrect or perhaps overstated.

Thirdly, Cyprian sometimes used allegorical interpretation even in places where his quotations seem text-based. In chapter seven of Unit. eccl, Cyprian proposes an unlikely allegorical interpretation of John 19:23: These are the words of Holy Scripture: Now as to His coat, because it was from the upper part woven throughout without a seam, they said to one another: Let us not divide it, but let us cast lots for it, whose it shall be. The ‘oneness’ with which He was clothed came ‘from the upper part,’ that is, from His Father in heaven, and could in no way be divided by whoever came to acquire it: it retained its well-knit wholeness indivisibly. That man cannot possess the garment of Christ, who rends and divides the Church of Christ.
A fourth problem lies in the fact that Latin copies of 1 John offer “support for a whole set of readings that have little or no attestation in Greek.” Brooke provides a listing of various “explanatory glosses” given by Augustine and Cyprian as well as some glosses found in the Speculum. Cyprian glosses the texts of 1 John 2:9; 2:16; and 4:3. Cyprian’s tendency to gloss the text combined with the problems evaluating patristic citations suggest the tentative possibility that: 1) Cyprian is the source of the Comma; or 2) Cyprian demonstrates the process that gave rise to it. The fact that a quotation is found in his writings does not necessarily mean it was drawn from the text of the New Testament.

A fifth problem concerns why Augustine, who lived at the time of the Arian controversy, did not bother to invoke the Comma in his writings. “Cyprian was the prime influence on North African Christianity from the period of his episcopate until the time of Augustine.” Cyprian’s writings were revered for over four centuries as one step below Scripture, and Augustine so revered Cyprian that he presented at least a dozen sermons celebrating a memorial feast to Cyprian. Nowhere in any of his writings does Augustine quote the Comma. Such a scenario is unlikely if Cyprian quoted the Comma.

1. After throuougly refuting this notion, he entertains the idea that Cyprian DID quote the verse and states: Therefore, even if Cyprian did quote the Comma then that merely moves the date of the first known quotation back a century. It does not change the fundamental reality: not one scrap of Greek evidence of the Comma exists in the first ten centuries, and all of the extant evidence comes from one secondary language, Latin.