The Johannine comma, as it is called, is a sequence of extra words in 1 John 5:7-8 which appear in some early printed Greek texts (notably those of Erasmus), later versions of the Latin Vulgate, and in the King James Version of the Bible. See these words below in italics in the KJV and the same verse from the newer ESV.
"For there are three that bear record (witness) in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the spirit, and the water, and the blood: and these three agree in one." -1 John 5:7-8, KJV
"For there are three that testify: the Spirit and the water and the blood; and these three agree." -1 John 5:7-8, ESV
Pre-16th century Greek manuscripts and translations
"These extra words are generally absent from the Greek manuscripts. In fact, they only appear in the text of four late medieval manuscripts. They seem to have originated as a marginal note added to certain Latin manuscripts during the middle ages, which was eventually incorporated into the text of most of the later Vulgate manuscripts." ^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." ^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 Erasmian promise
"Erasmus promised that he would insert the Comma Johanneum, as it is called, in future editions if a single Greek manuscript could be found that contained the passage. At length such a copy was found—or made to order." (Bruce Metzger)
However, on pg 291 (n2) of the (new) 3rd edition of The Text of the New Testament Bruce Metzger writes:
"What is said on p. 101 above about Erasmus' promise to include the Comma Johanneum if one Greek manuscript were found that contained it, and his subsequent suspicion that MS. 61 was written expressly to force him to do so, needs to be corrected in the light of the research of H.J. de Jonge, a specialist in Erasmian studies who finds no explicit evidence that supports this frequently made assertion; see his "Erasmus and the Comma Johanneum", Ephemerides Theologicae Lovanienses, lvi (1980)," pp 381-9.
In A History of the Debate over 1 John 5:7,8, Michael Maynard records that H.J. de Jonge, the Dean of the Faculty of Theology at Rijksuniversiteit (Leiden, Netherlands), a recognized specialist in Erasmian studies, refuted the myth of a promise in 1980, stating that Metzger's view on Erasmus' promise "has no foundation in Erasmus' work. Consequently it is highly improbable that he included the difficult passage because he considered himself bound by any such promise."
In a letter of June 13, 1995, to Maynard, de Jonge wrote:
"I have checked again Erasmus' words quoted by Erika Rummel and her comments on them in her book Erasmus' Annotations. This is what Erasmus writes [on] in his Liber tertius quo respondet ... Ed. Lei: Erasmus first records that Lee had reproached him with neglect of the MSS. of 1 John because Erasmus (according to Lee) had consulted only one MS. Erasmus replies that he had certainly not used only one ms., but many copies, first in England, then in Brabant, and finally at Basle. He cannot accept, therefore, Lee's reproach of negligence and impiety. 'Is it negligence and impiety, if I did not consult manuscripts which were simply not within my reach? I have at least assembled whatever I could assemble. Let Lee produce a Greek MS. which contains what my edition does not contain and let him show that that manuscript was within my reach. Only then can he reproach me with negligence in sacred matters.'
"From this passage you can see that Erasmus does not challenge Lee to produce a manuscript etc. What Erasmus argues is that Lee may only reproach Erasmus with negligence of MSS if he demonstrates that Erasmus could have consulted any MS. in which the Comma Johanneum figured. Erasmus does not at all ask for a MS. containing the Comma Johanneum. He denies Lee the right to call him negligent and impious if the latter does not prove that Erasmus neglected a manuscript to which he had access.
"In short, Rummel's interpretation is simply wrong. The passage she quotes has nothing to do with a challenge. Also, she cuts the quotation short, so that the real sense of the passage becomes unrecognizable. She is absolutely not justified in speaking of a challenge in this case or in the case of any other passage on the subject" (emphasis in original) (de Jonge, cited from A History of the Debate over 1 John 5:7,8, Michael Maynard, p. 383).