As a newbie, I probably shouldn't reply. But I do have a copy of the Orthodox New Testament, published by the Holy Apostles Convent in Buena Vista, Colorado. It puts the disputed part of 1 John 5:7, 8 in italics: "For there are three bearing witness in the heaven: the Father, the Logos, and the Holy Spirit; and these Three are One. And there are three bearing witness in the earth: the Spirit, and the water, and the blood; and these three are for the one."
A note for the passage says: "These words--'in the heaven: the Father, the Logos (Word), and the Holy Spirit; and these Three are One. And there are three bearing witness i the earth"--are not found in any early Greek manuscript. Four of eight existing manuscripts contain the passage in what appears to be a translation from the late recension of the Latin Vulgate. The four minuscules which contain the passage as a variant reading written in the margin as a later addition are as follows: Minuscules 88 v.l. (16th c.), 221 v.1. (10th c.), 429 v.1. (16th c.), and 636 v. 1. (16thc.). The remaining Minuscules are: 61 (16th c.), 629 (14th c.), 918 (16th c.), 2318 (18th c.).
"Down to the thirteenth century, no Greek writer makes mention of this passage. Later, it is cited only by one Greek author, the 15th-century Dominican monk and adherent of Thomas Aquinas, Manuel Calecas. The first to cite this phrase in Latin was the Spanish heresiarch Priscillian (4th c.), and it appears in a considerable number of Latin manuscripts. It entered the printed versions of the Greek text when it was included as a translation from the Latin in the first printed Greek Bible edition (1514), by the publisher Cardinal Ximenes de Cisneros. The Greek Lectionaries of Apostolike Diakonia and Phos include it. The Constantinople Edition, designating the phrase as an addition, has printed it in small italic type.
"it is absent from the manuscripts of all ancient versions (Syriac, Coptic, Armenian, Ethiopic, Arabic, Slavonic), except the Latin. However, it is neither found in the Old Latin, nor quoted by St. Cyprian or Augustine, nor is it in the Vulgate issued by Blessed Jerome" (explanatory note 39).
As a believer in the Trinity, I've long been partial to the verse, but I concede that it's poorly supported by the extant manuscripts. So it's not useful for refuting Unitarians or Jehovah's Witnesses. Still, the formula "Father, Word, and Holy Spirit"--instead of the usual "Father, Son, and Holy Spirit"--sounds so Johannine that I like to see the verse retained with the appropriate explanatory notes.