From A Textual Commentary on the Greek New Testament, by Bruce Metzger:
5.78 , 8 {A}
After the Textus Receptus adds the following: , , , . . (8) . 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 eight, and these contain the passage in what appears to be a translation from a late recension of the Latin Vulgate. Four of the eight manuscripts contain the passage as a variant reading written in the margin as a later addition to the manuscript. The eight manuscripts are as follows:
61: codex Montfortianus, dating from the early sixteenth century.
88v.r.: a variant reading in a sixteenth century hand, added to the fourteenthcentury codex Regius of Naples.
221v.r. a variant reading added to a tenthcentury manuscript in the Bodleian Library at Oxford.
429v.r.: a variant reading added to a sixteenthcentury manuscript at Wolfenbüttel.
636v.r.: a variant reading added to a sixteenthcentury manuscript at Naples.
918: a sixteenthcentury manuscript at the Escorial, Spain.
2318: an eighteenthcentury manuscript, influenced by the Clementine Vulgate, at Bucharest, Rumania.
(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.d. 541-46] and codex Amiatinus [copied before a.d. 716]) or (c) as revised by Alcuin (first hand of codex Vallicellianus [ninth century]).
The earliest instance of the passage being quoted as a part of the actual text of the Epistle 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. In the fifth century the gloss was quoted by Latin Fathers in North Africa and Italy as part of the text of the Epistle, and from the sixth century onwards it is found more and more frequently in manuscripts of the Old Latin and of the Vulgate. In these various witnesses the wording of the passage differs in several particulars. (For examples of other intrusions into the Latin text of 1John, see 2.17; 4.3; 5.6, and 20.)
(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 1John, or Metzger, The Text of the New Testament, pp. 101f.; cf. also Ezra Abbot, I. John v. 7 and Luthers German Bible, in The Authorship of the Fourth Gospel and Other Critical Essays (Boston, 1888), pp. 458463.