The Johannine Gloss or Johannine Comma, as it is more commonly known, is an interpolated passage which appears in 1 John 5:7-8, shown here in brackets: "For there are three who bear witness [in heaven: the Father, the Word, and the Holy Spirit; and these three are one. And there are three that bear witness on earth]: the Spirit, and the water, and the blood; and these three are one."
The New Catholic Encyclopedia explains that
the bracketed phrases appear in the [Vulgate] version of the Bible, the official version of the Sacred Scriptures for the Latin Rite of the Church. Among scholars these phrases are commonly called the "Johannine Comma." On the basis of manuscript evidence scholars seriously question their authenticity. The Comma is absent in all the ancient Greek manuscripts of the New Testament with the exception of four rather recent manuscripts that date from the thirteenth to the sixteenth centuries.
The Comma is lacking in such ancient Oriental versions as the Peshitta, Philoxenian, Coptic, Ethiopic, and Armenian. While the majority of the Latin manuscripts of 1 John do contain the Comma, the earlier and better manuscripts, both the Old Latin and the Vulgate versions, lack it. The earliest manuscript in which it appears dates from the ninth century.
The Fathers of the East do not quote or refer to the Johannine Comma in their Christological controversies. This omission indicates that the Comma was not part of the biblical text of their time, for they surely would have used it had it been in the text. Some fourth-century Latin writers, while referring to 1 John 5:8b and giving this a Trinitarian interpretation, failed to give any indication that they knew of the existence of the Comma as a scriptural passage.
Due to the overcritical spirit that was prevalent in the late nineteenth and early twentieth centuries, the Church considered it necessary in its decree of the Holy Office of January 13,1897 to caution its scholars against rashly rejecting or doubting the authenticity of this passage. However, in a decree of June 2, 1927, the Holy Office clarified its earlier statement in declaring that scholars may be inclined to doubt or reject the authenticity of the Johannine Comma subject to any forthcoming judgment of the Church. No scholar any longer accepts its authenticity. But even though the Comma is not a biblical passage, it is a firm witness to the fact that the faith of the [early] Christian was fully Trinitarian.