The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
The passage is called the Johannine Comma and is not found in the majority of Greek manuscripts.  However, the verse is a wonderful testimony to the Heavenly Trinity and should be maintained in our English versions, not only because of its doctrinal significance but because of the external and internal evidence that testify to its authenticity.
The External Support: Although not found in most Greek manuscripts, the Johannine Comma is found in several. It is contained in 629 (fourteenth century), 61 (sixteenth century), 918 (sixteenth century), 2473 (seventeenth century), and 2318 (eighteenth century). It is also in the margins of 221 (tenth century), 635 (eleventh century), 88 (twelveth century), 429 (fourteenth century), and 636 (fifteenth century). There are about five hundred existing manuscripts of 1 John chapter five that do not contain the Comma.  It is clear that the reading found in the Textus Receptus is the minority reading with later textual support from the Greek witnesses. Nevertheless, being a minority reading does not eliminate it as genuine. The Critical Text considers the reading Iesou (of Jesus) to be the genuine reading instead of Iesou Christou (of Jesus Christ) in 1 John 1:7. Yet Iesou is the minority reading with only twenty-four manuscripts supporting it, while four hundred seventy-seven manuscripts support the reading Iesou Christou found in the Textus Receptus. Likewise, in 1 John 2:20 the minority reading pantes (all) has only twelve manuscripts supporting it, while the majority reading is panta (all things) has four hundred ninety-one manuscripts. Still, the Critical Text favors the minority reading over the majority in that passage. This is common place throughout the First Epistle of John, and the New Testament as a whole. Therefore, simply because a reading is in the minority does not eliminate it as being considered original.
While the Greek textual evidence is weak, the Latin textual evidence for the Comma is extremely strong. It is in the vast majority of the Old Latin manuscripts, which outnumber the Greek manuscripts. Although some doubt if the Comma was a part of Jerome's original Vulgate, the evidence suggests that it was. Jerome states:
In that place particularly where we read about the unity of the Trinity which is placed in the First Epistle of John, in which also the names of three, i.e. of water, of blood, and of spirit, do they place in their edition and omitting the testimony of the Father; and the Word, and the Spirit in which the catholic faith is especially confirmed and the single substance of the Father, the Son and the Holy Spirit is confirmed. 
Other church fathers are also known to have quoted the Comma. Although some have questioned if Cyprian (258 AD) knew of the Comma, his citation certainly suggests that he did. He writes: "The Lord says, 'I and the Father are one' and likewise it is written of the Father and the Son and the Holy Spirit, 'And these three are one'."  Also, there is no doubt that Priscillian (385 AD) cites the Comma:
As John says "and there are three which give testimony on earth, the water, the flesh, the blood, and these three are in one, and there are three which give testimony in heaven, the Father, the Word, and the Spirit, and these three are one in Christ Jesus." 
Likewise, the anti-Arian work compiled by an unknown writer, the Varimadum (380 AD) states: "And John the Evangelist says, . . . 'And there are three who give testimony in heaven, the Father, the Word, and the Spirit, and these three are one'."  Additionally, Cassian (435 AD), Cassiodorus (580 AD), and a host of other African and Western bishops in subsequent centuries have cited the Comma.  Therefore, we see that the reading has massive and ancient textual support apart from the Greek witnesses.
Internal Evidence: The structure of the Comma is certainly Johannine in style. John is noted for referring to Christ as "the Word." If 1 John 5:7 were an interpretation of verse eight, as some have suggested, than we would expect the verse to use "Son" instead of "Word." However, the verse uses the Greek word logos, which is uniquely in the style of John and provides evidence of its genuineness. Also, we find John drawing parallels between the Trinity and what they testify (1 John 4:13-14). Therefore, it comes as no surprise to find a parallel of witnesses containing groups of three, one heavenly and one earthly.
The strongest evidence, however, is found in the Greek text itself. Looking at 1 John 5:8, there are three nouns which, in Greek, stand in the neuter (Spirit, water, and blood). However, they are followed by a participle that is masculine. The Greek phrase here is oi marturountes (who bare witness). Those who know the Greek language understand this to be poor grammar if left to stand on its own. Even more noticeably, verse six has the same participle but stands in the neuter (Gk.: to marturoun). Why are three neuter nouns supported with a masculine participle? The answer is found if we include verse seven. There we have two masculine nouns (Father and Son) followed by a neuter noun (Spirit). The verse also has the Greek masculine participle oi marturountes. With this clause introducing verse eight, it is very proper for the participle in verse eight to be masculine, because of the masculine nouns in verse seven. But if verse seven were not there it would become improper Greek grammar.
Even though Gregory of Nazianzus (390 AD) does not testify to the authenticity of the Comma, he makes mention of the flawed grammar resulting from its absence. In his Theological Orientations he writes referring to John:
. . . (he has not been consistent) in the way he has happened upon his terms; for after using Three in the masculine gender he adds three words which are neuter, contrary to the definitions and laws which you and your grammarians have laid down. For what is the difference between putting a masculine Three first, and then adding One and One and One in the neuter, or after a masculine One and One and One to use the Three not in the masculine but in the neuter, which you yourselves disclaim in the case of Deity? 
It is clear that Gregory recognized the inconsistency with Greek grammar if all we have are verses six and eight without verse seven. Other scholars have recognized the same thing. This was the argument of Robert Dabney of Union Theological Seminary in his book, The Doctrinal Various Readings of the New Testament Greek (1891). Bishop Middleton in his book, Doctrine of the Greek Article, argues that verse seven must be a part of the text according to the Greek structure of the passage. Even in the famous commentary by Matthew Henry, there is a note stating that we must have verse seven if we are to have proper Greek in verse eight. 
While the external evidence makes the originality of the Comma possible, the internal evidence makes it very probable. When we consider the providential hand of God and His use of the Traditional Text in the Reformation it is clear that the Comma is authentic.
 The first and second editions of Erasmus' Greek text did not contain the Comma. It is generally reported that Erasmus promised to include the Comma in his third edition if a single manuscript containing the Comma could be produced. A Franciscan friar named Froy (or Roy) forged a Greek text containing it by translating the Comma from the Latin into Greek. Erasmus was then presented with this falsified manuscript and, being faithful to his word, reluctantly included the Comma in the 1522 edition. However, as has now been admitted by Dr. Bruce Metzger, this story is apocryphal (The Text Of The New Testament, 291). Metzger notes that H. J. de Jonge, a respected specialist on Erasmus, has established that there is no evidence of such events occurring. Therefore, opponents of the Comma in light of the historical facts should no longer affirm this report.
 Kurt Aland, in connection with Annette Benduhn-Mertz and Gerd Mink, Text und Textwert der griechischen Handschriften des Neuen Testaments: I. Die Katholischen Briefe Band 1: Das Material (Berlin: Walter De Gruyter, 1987), 163-166.
 Prologue To The Canonical Epistles. The Latin text reads, "si ab interpretibus fideliter in latinum eloquium verterentur nec ambiguitatem legentibus facerent nec trinitatis unitate in prima joannis epistola positum legimus, in qua etiam, trium tantummodo vocabula hoc est aquae, sanguinis et spiritus in ipsa sua editione ponentes et patris verbique ac aspiritus testimoninum omittentes, in quo maxime et fides catholica roboratur, et patris et filii et spirtus sancti una divinitatis substantia comprobatur."
 Treatises 1 5:423.
 Liber Apologeticus.
 Varimadum 90:20-21.
 Some other sources include the Speculum (or m of 450 AD), Victor of Vita (489 AD), Victor Vitensis (485 AD), Codex Freisingensis (of 500 AD), Fulgentius (533 AD), Isidore of Seville (636 AD), Codex Pal Legionensis (650 AD), and Jaqub of Edessa (700 AD). Interestingly, it is also found in the edition of the Apostle's Creed used by the Waldenses and Albigensians of the twelfth century.
 Fifth Orientation the Holy Spirit.
 Actually the 1 John commentary is the work of "Mr. John Reynolds of Shrewsbury," one of the ministers who completed Matthew Henry's commentary, which was left incomplete [only up to the end of Acts] at Henry's death in 1714.
"Seek ye out of the book of the Lord, and read" —Isaiah 34:16, KJV
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