In the King James Version and NKJV (as below), 1 John 5:6-8 reads as follows:
This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. For there are three that 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, the water, and the blood; and these three agree as one.
All other modern translations, however, read as follows:
This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. For there are three that bear witness: the Spirit, the water, and the blood; and these three agree as one.
As can be seen, modern translations omit the words bolded above. The difference is due to the fact the KJV (and the NKJV, as a revision of the KJV) is based on a Greek text that has come to be known as the Textus Receptus, which was originally compiled on the basis of a few late Byzantine manuscripts and subsequently edited and improved. Most modern translation are based on the error-laden and inferior Greek text various referred to as the Nestle-Aland text, the United Bible Societies text, the NU, or the “critical text.”
However, in this case the choice is not between the NU text and the Majority (Byzantine) Text reading, as most of the Byzantine manuscripts also omit the words in question. We have some 480 manuscripts extant in 1 John 5, and only five of them, the earliest (minuscule 629) from the 14th century, contain these words. (Another six were originally missing the words but had them added into the margin at later dates.)
Now, as we have shown elsewhere, the chances of an erroneous reading (in this case, the omission of those words if they were the autograph) ever infecting the large majority of manuscripts is vanishingly small, so prima facie we should conclude that these words were not part of the original text. Given that they are found in some Latin manuscripts that are far earlier, it is possible that the words were somehow added there and became part of the Latin tradition and eventually moved into a few of the Greek manuscripts.
However, it is not the case that no defence of the words can be made. Consider the Greek text (the words in question are bolded):
οὗτός ἐστιν ὁ ἐλθὼν δι᾽ ὕδατος καὶ αἵματος, Ἰησοῦς Χριστός οὐκ ἐν τῷ ὕδατι μόνον, ἀλλ᾽ ἐν τῷ ὕδατι καὶ τῷ αἵματι καὶ τὸ πνεῦμά ἐστιν τὸ μαρτυροῦν, ὅτι τὸ πνεῦμά ἐστιν ἡ ἀλήθεια ὅτι τρεῖς εἰσιν οἱ μαρτυροῦντες ἐν τῷ οὐρανῷ, ὁ πατὴρ, ὁ λόγος, καὶ τὸ ἅγιον πνεῦμα καὶ οὗτοι οἱ τρεῖς ἕν εἰσιν καὶ τρεῖς εἰσιν οἱ μαρτυροῦντες ἐν τῇ γῇ, τὸ πνεῦμα, καὶ τὸ ὕδωρ, καὶ τὸ αἷμα καὶ οἱ τρεῖς εἰς τὸ ἕν εἰσιν
Now, notice the word οἱ (underlined); it is the masculine plural article used as a substantive. With the words in question present, οἱ refers to the Father, the Son, and the Holy Spirit. Father and Son are both masculine nouns in the Greek (Spirit is neuter, but is elsewhere referred to in the masculine, being a Person), so we would expect the masculine plural article.
But without the words in question, οἱ refers to “the Spirit, the water, and the blood,” all of which are neuter nouns, so we would expect the neuter plural of the article (τὰ) to be used here.
Next, consider the bolded and underlined phrase οὗτοι οἱ τρεῖς ἕν εἰσιν from the words in question: “these three are one”; notice that there is no article with “ἕν,” as it is not referring to a “one” that has previously been specified. Yet if we look at the last phrase in the quote above, “οἱ τρεῖς εἰς τὸ ἕν εἰσιν” “these three agree as one,” we see that the article is present with “one” (τὸ ἕν, literally “the one”), which suggests the reference is to a “one” that has previously been noted; that is the case if the words in question are present, but there is no “one” previously noted if those words are absent.
In sum, then, the absence of these words in almost all Greek manuscripts strongly indicates that they were not included in the autograph. However, it is not absolutely impossible that a minority reading could be correct, and the grammar of the Greek here indicates that these words just might indeed be part of the original autograph.
Sources
1. For details about these matters, see Tors, John. “A Primer on New Testament Textual Criticism (in Manageable, Bite-Sized Chunks)”
2. “Johannine Comma (1 John 5:7).” Posted at http://www.kjvtoday.com/home/the-father-the-word-and-the-holy-ghost-in-1-john-57
3. Tors, “A Primer on New Testament Textual Criticism,” op. cit.