Dr. J.A. Moorman - a dedicated Godly minister, capsuling the posture of modern textual criticism which insists upon the omission of the passage, has set forth the following particulars:1
2. The passage is not quoted by any of the Greek Fathers who would have employed it as proof in the Trinitarian controversies (Arian and Sabellian) had they known of the section. Its first appearance in Greek is in a Greek version of the (Latin) Acts of the Latern Council in 1215.
3. The section is not present in the mss of all the ancient versions except the Latin. Even then, it is not found in the Old Latin in its early form and it is not in Jerome's Vulgate (c.405). The earliest instance of the passage being quoted as part of the actual text of First John is a fourth century Latin treatise. Supposedly the "gloss" arose when the original passage was understood to symbolize the Trinity (through the mention of three witnesses; the Spirit, and the water and the blood). This interpretation, they tell us, may have been written as a marginal note at first and, as time went on, found its way into the text. The "gloss" was quoted by Latin Fathers in North Africa and Italy in the 5th century as part of the text of the Epistle. From the 6th century on, it is found more and more frequently in mss of the Old Latin and Vulgate.
4. If the passage were original, a compelling reason or reasons should have been found to account for its omission, either accidently or deliberately, by all of the copyists of hundreds of Greek mss and by translators of ancient versions (called "transcriptional probability" - p. 1). Lastly, they inform us that the passage makes an awkward break in the sense (called "intrinsic probability" - p. 1).