“Is I John 5:7 a sufficient source in defending the Trinity?”
There is more to this question than meets the eye. First, one text is sufficient proof to teach any doctrine. The question is, “Does 1 John 5:7 teach the Trinity, and does it do so sufficiently and clearly to be used as a proof text of that doctrine?” Second, the question is, “Does 1 John 5:7 belong in the New Testament?”
The first question concerns exegesis or the interpretation of Scripture. The second question concerns textual criticism or the study of the manuscripts in order to determine the actual words of Scripture.
The more controversial question on textual criticism should be answered first.
The KJV reads thus:
(6) This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. (7) For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (8) And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one (1 John 5:6-8).
The vast majority of modern versions produced after the KJV have a shorter reading, omitting verse 7 almost entirely from the text.
The shorter reading, represented, for example, in the NIV, is as follows:
(6) This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. (7) For there are three that testify: (8) the Spirit, the water and the blood; and the three are in agreement (1 John 5:6-8).
In the NIV, the “heavenly witnesses” (“the Father, the Word and the Holy Ghost”) vanish from the Bible, while the “earthly witnesses” (“the Spirit, the water and the blood”) remain, although even the words “in earth” (v. 8) are omitted in the NIV.
Modern Bible versions omit other passages as well, or question their authenticity and authority in misleading footnotes. Examples of these are John 5:4 (the stirring of the water of the pool of Bethesda), John 8:1–11 (the adulteress whom Jesus pardoned), and Mark 16:9–20 (the so-called “longer ending of Mark”). Many modern Bible versions make these passages with footnotes, “The best and earliest manuscripts do not contain these words” or “Many ancient witnesses omit these words,” or “The most reliable and accurate manuscripts have a different reading,” and the like.
When you see such footnotes, recognize the bias behind them: certain manuscripts are supposedly the best, the most reliable, or the most accurate because they are the oldest or most ancient. The age of a manuscript is a fact (as far as they can be dated accurately). The quality, reliability, or accuracy of a manuscript is an opinion.
Textual criticism is an issue because we no longer possess the autographs, that is, the actual pages or parchments on which John, Peter or Paul wrote the words of scripture. We only have copies, or copies of copies of the original. This does not mean, however, that we do not have the text of scripture. Any misuse of textual criticism in order to undermine the authority of scripture must be vigorously rejected. We have the very words that the Holy Spirit moved holy men to write (2 Peter 1:21). A plethora of modern Bible versions with their misleading footnotes has eroded the confidence of God’s children in God’s word and has confused many of the saints. This is not only regrettable; it is indefensible and deplorable.
During the copying process, mistakes occurred, most of which were very minor: variations of spelling, the inclusion/omission of conjunctions (“and,” “but,” “for,” etc.), slight differences in word order (“Christ Jesus” vs. “Jesus Christ,” for example). In cases where scribes miscopied a text, the original text can still be determined. It is important to note that no doctrine of the word of God is affected by these textual variants. Moreover, there is more manuscript evidence for the Bible than for any ancient text. There are some 5,000 manuscripts of the New Testament, although only about 200 contain the whole New Testament. Besides the many manuscripts, we have huge numbers of quotations from the New Testament from the early church fathers, as well as from ancient translations of the Scriptures (Syriac, Coptic, Latin, etc.), which enable us to “reconstruct” the original Greek words of the New Testament.
Textual critics identify different kinds or families of manuscripts. A very rough division is as follows. The first is the Majority Text (MT), which is the basis for the Textus Receptus, or the Received Text, which underlies the KJV. The second is the NU Text (The Nestle Aland or United Bible Societies Text), or critical text, which relies heavily upon the Alexandrian texts, which are favored by many textual critics because of their supposed greater antiquity. Some of these ancient and favored texts are Aleph (Codex Sinaiticus, discovered in a monastery in the 1800’s), B (Codex Vaticanus, discovered in the Vatican library in 1480), A (Codex Alexandrinus), D (Codex Bezae) and C (Codex Ephraemi Rescriptus or Ephraim).
For centuries the church used a Bible containing 1 John 5:7 (and the other disputed texts). Certain manuscripts, many of them discovered later, omit such verses. Because these manuscripts are supposedly more ancient and superior, modern Bible versions began to be published without the verses.
But the question is, “Why do these ancient manuscripts not contain these verses?” Why, for example, does the text received by the church (the Textus Receptus) differ from the text favored by scholars? Why is the MT set aside in favor of Sinaiticus, Vaticanus, and other “ancient witnesses”? One plausible answer is this: the church did not use these ancient manuscripts because it did not favor them. The manuscripts which the church did use and which were circulated in the church wore out and were replaced by “newer” copies, while the unapproved (and even corrupted) manuscripts lay unused until scholars discovered them. You may have several Bibles (I certainly do). Which Bible wears out more quickly? The one you use. Which Bible is still in pristine condition on your bookshelf? The one you do not use.
One thing many textual critics ignore (and this is certainly true of the unbelieving critics) is the doctrine of scripture’s preservation. God not only inspired men to write his word (2 Tim. 3:16; 2 Peter 1:20–21), but he also preserved the text of scripture in his providence so that in all ages the church has the word of God. God’s word does not belong to critics and scholars; it belongs to God’s people, the church. That is where the Bible is used, read, studied, propagated, and preserved.
Consider the Westminster Confession of Faith:
The Old Testament in Hebrew (which was the native language of the people of God of old) and the New Testament in Greek (which at the time of the writing of it was most generally known to the nations), being immediately inspired by God and by his singular care and providence kept pure in all ages are therefore authentical; so as in all controversies of religion the Church is finally to appeal unto them but because these original tongues are not known to all the people of God who have right unto and interest in the Scriptures and are commanded, in the fear of God, to read and search them, therefore they are to be translated into the vulgar language of every nation unto which they come, that the word of God dwelling plentifully in all, they may worship him in an acceptable manner, and through patience and comfort of the Scriptures may have hope (The Westminster Confession of Faith, Article 1, paragraph 8, italics added).
I John 5:7 is the most difficult textual variant to explain, mainly because there is no record of it in any Greek manuscript dated before the fourteenth century. It is also very difficult to prove with any certainty that the early church fathers quoted it. It is possible that the text dropped out of the Greek manuscripts very early in a scribal error where the phrase translated “there are three that bear record” (v. 7) and “there are three that bear witness” (v. 8) is the same in the Greek. Such a scribal error was not corrected in later copies. However, it is also possible that the text was preserved in the church through Latin translations. This is the view of Edward Hills:
On the basis of the external evidence, it is at least possible that the Johannine comma is a reading that somehow dropped out of the Greek New Testament, but was preserved in the Latin text through the usage of the Latin-speaking Church, and this possibility grows more and more toward probability as we consider the internal evidence.
I apologize for the overly technical nature of this response. For further information, the reader is advised to consult the material of the Trinitarian Bible Society.
Although we might like to have one text to prove the Trinity, no such text exists. The Bible teaches the Trinity, of course, but it does not do so by means of one proof text. Certain texts such as Matthew 28:19 and 2 Corinthians 13:14 come close to teaching the Trinity, but we believe the Trinity on the basis of the testimony of the entirety of God’s word.
1 John 5:7 (“and these three are one”) must be explained. What is the meaning of “one”? Is “one” oneness of being, of person, or of will? If oneness of being is the meaning, 1 John 5:7 teaches the Trinity, that the Father, the Word (the Son), and the Spirit are one God. If oneness of person is the meaning, 1 John 5:7 could be appealed to in favor of Sabellianism or Modalism, a heresy that asserts that the Father, the Son and the Spirit are modes or faces of one person, but not distinct persons. Therefore, the Father, according to Sabellianism, is the same person as the Son. (Sabellianism was as great a threat to the early church, as was Arianism, and the danger of Sabellianism may explain the reluctance of the fathers to appeal to 1 John 5:7 as evidence for the Trinity). If oneness of will is the meaning, the text teaches that the Father, the Son (Word), and the Spirit agree in their testimony without addressing the question of the relationship between the persons. That is the meaning of oneness in verse 8, “and these three agree in one.” Whatever the precise meaning of 1 John 5:7, it clearly does not teach heresy (such as Sabellianism), and it must be compared with other passages of the word of God.
The short answer is that 1 John 5:7 does not unambiguously teach the Trinity and as a stand-alone text is not a sufficient source for the defense of that doctrine. Nevertheless, the Lord has wonderfully preserved his word, including this text in 1 John 5:7, and the Bible abundantly teaches the Trinity throughout.
 The New King James Version includes the words in the text, but adds this footnote, “NU-Text and M-Text omit the words from in heaven (verse 7) through on earth (verse 8). Only four or five very late manuscripts contain these words in Greek.”
 The New International Footnote reads, “Late manuscripts of the Vulgate testify in heaven: the Father, the Word and the Holy Spirit, and these three are one. 8 And there are three that testify on earth: the (not found in any Greek manuscript before the fourteenth century).”
 Edward F. Hills, The King James Version Defended (The Christian Research Press: DeMoines, IA, repr. 2000), p. 210.
 Why I John 5:7-8 Is In The Bible (G.W. and D.E. Anderson), Trinitarian Bible Socety (TBS), http://www.tbsbibles.org/pdf_information/40-1.pdf . Another useful article, although not from TBS, is http://www.studytoanswer.net/bibleversions/1john5n7.html . Moreover, Matthew Henry comments extensively on I John 5:7 in his commentary, which the reader is advised to read carefully.