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This text is not found in any modern English versions, i.e., not found in those versions that are translated from the current representations of the original texts. (The NKJV is not such a modern translation. It is still totally dependent upon the KJV and the same faulty Hebrew and Greek texts from which the KJV was translated. )
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For additional details regarding the corruption of the Textus Receptus that was the basis of the Greek text used by the KJV translators, browse [bibletexts.com/kjv-tr.htm#1jo0507](http://www.bibletexts.com/kjv-tr.htm#1jo0507). Below is an excerpt from that article.
Even up to the fifth and final edition of Erasmus' Greek text in 1535, Erasmus fell prey to pressure and manipulation from church authorities to add to subsequent editions phrases and entire verses that he strongly (and rightly) suspected were not part of the original text. (Ibid., pages 100-101, which document how Erasmus was conned to include what is translated in the KJV in 1Jo 5:7-8, the following text: "in heaven, the Father, the Word, and the Holy Ghost; and these three are one. And there are three that bear witness in earth." Conservative biblical scholar F.F. Bruce (History of the English Bible, Third Edition, New York: Oxford University Press, 1978, pages 141-142) explains the sad history of how 1Jo 5:7-8 had been errantly added to Erasmus' Greek text:
The words "in heaven, the Father, the Word, and the Holy Ghost; and these three are one. And there are three that bear witness in earth."] omitted in the R.V. [Revised Version, 1881] were no part of the original Greek text, nor yet of the Latin Vulgate in its earliest form. They first appear in the writings of a Spanish Christian leader named Priscillian, who was executed for heresy in A.D. 385. Later they made their way into copies of the Latin text of the Bible. When Erasmus prepared his printed edition of the Greek New Testament, he rightly left those words out, but was attacked for this by people who felt that the passage was a valuable proof-text for the doctrine of the Trinity. He replied (rather incautiously) that if he could be shown any Greek manuscript which contained the words, he would include them in his next edition. Unfortunately, a Greek manuscript not more than some twenty years old was produced in which the words appeared: they had been translated into Greek from Latin. Of course, the fact that the only Greek manuscript exhibiting the words belonged to the sixteenth century was in itself an argument against their authenticity, but Erasmus had given his promise, and so in his 1522 edition he included the passage. (To-day one or two other very late Greek manuscripts are known to contain this passages; all others omit it.)