Here is a nice summary of the case against the Johannine Comma:
“The added text of the KJV and NKJV, known as the “Johannine Comma” (Comma Johanneum), lacks any significant textual evidence. What evidence there is is very late. Therefore, it should not be included as original. No Greek text before the thirteenth century contains these words and, after that, only Latin texts until Erasmus’ third edition (sixteenth century). It appears in only eight extant Greek MSS, with four of them being marginal notes. . .It is also absent from all of the non-Latin or Greek MSS prior to AD 1500, including the Arabic, Armenian, Coptic, Ethiopic, Slavonic, and Syriac translations. As Brown (777) notes, this is “an incredible situation if it were once part of the original Greek text of 1 John.” The earliest Latin MSS also lack the Comma. In 1592, it was added into the Catholic edition of the Latin Vulgate. . .It appears to have a Spanish origin in the late fourth century, possibly written by the bishop Instantius, a follower of Priscillian, who was martyred around AD 386. . .The words were clearly added to support Trinitarian theology. For the history of how it was forced into Erasmus’ third Greek edition and, subsequently, into the KJV, Brown’s (779-80) summary is worth reading.”
From: 1, 2 & 3 John by Gary W. Derickson; Evangelical Exegetical Commentary series (Bellingham, WA: Lexham Press, 2014), p. 513.