1 John 5:7–8 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one (KJV).
1. Many English versions have a shorter rendition of 1 John 5:7 and 8 than the KJV quoted above. The King James Version has words that support the Trinity that most modern versions do not have. How can this be? The reason that there are different translations of this verse is that some Greek texts contain an addition that was not in the original, and that addition was placed into some English versions, such as the KJV (the words added to some Greek texts are underlined in the quotation above).
The note in the NIV Study Bible, which is well known for its ardent belief in the Trinity, says, “The addition is not found in any Greek manuscript or NT translation prior to the 16th century.”
Most modern versions are translated from Greek texts without the addition. We will quote the NIV: “For there are three that testify: the Spirit, the water and the blood; and the three are in agreement.” We agree with the textual scholars and conclude from the evidence of the Greek texts that the statement that the Father, the Word and the Holy Spirit are “one” was added to the Word of God by men, and thus has no weight of truth. Below are various bible translations for 1 John 5:7 & 8
There are many Trinitarian scholars who freely admit that the Greek text from which the KJV is translated was adjusted in this verse to support the Trinity. The Greek scholar A. T. Robertson, author of the unparalleled work, A Grammar of the Greek New Testament in Light of Historical Research, and the multi-volumed Word Pictures in the New Testament, writes:
At this point [1 John 5:7] the Latin Vulgate gives the words in the Textus Receptus, found in no Greek MS. save two late cursives (162 in the Vatican Library of the fifteenth century, [No.] 34 of the sixteenth century in Trinity College, Dublin). Jerome did not have it. Erasmus did not have it in his first edition, but rashly offered to insert it if a single Greek MS. had it, and 34 was produced with the insertion, as if made to order. Some Latin scribe caught up Cyprian’s exegesis and wrote it on the margin of his text, and so it got into the Vulgate and finally into the Textus Receptus by the stupidity of Erasmus.” 
Robertson shows how this addition entered the text. It was a marginal note. Since all texts were hand-copied, when a scribe, copying a text, accidentally left a word or sentence out of his copy, he would place it in the margin in hopes that the next scribe would copy it back into the text.
Unfortunately, scribes occasionally did not make the distinction between what a previous scribe had left out of the last copy and wrote in the margin, and marginal notes that another scribe had written in the margin to help him understand the text. Therefore, some marginal notes got copied into the text as Scripture. Usually these additions are easy to spot because the “new” text will differ from all the other texts.
However, there are times when people adore their theology more than the God-breathed original, and they fight for the man-made addition as if it were the original words of God. This has been the case with 1 John 5:7 and 8, and we applaud the honesty of the translators of modern versions who have left it out of their translations.
The famous textual scholar, F. F. Bruce, does not even mention the addition in his commentary on 1 John (The Epistles of John). The International Critical Commentary does not mention it either. The conservative commentator R. C. H. Lenski, in his 12 volume commentary on the New Testament, only mentions that it is proper to leave the addition out. He writes: “The R. V. [Revised Version] is right in not even noting in the margin the interpolation found in the A.V. [KJV].” Henry Alford, author of the The Greek Testament, a Greek New Testament with extensive critical notes and commentary, writes:
…OMITTED BY ALL GREEK MANUSCRIPTS previous to the beginning of the 16th century; 
2. With the spurious addition gone, it is clear that there is no reference to the Trinity in 1 John 5:7 and 8. The context is speaking of believing that Jesus is the Son of God (v. 5 and 10). There are three that testify that Jesus is the Son of God: the spirit that Jesus received at his baptism, the water of his baptism and the blood that he shed.
Scripture says, “We accept man’s testimony, but God’s testimony is greater because it is the testimony of God which He has given about his Son” (v. 9). This verse is so true! How often people accept man’s testimony and believe what men say, but do not believe what God says. We need to accept the testimony of God that He has given about His Son, and agree with the testimony of the spirit, the water and the blood, that Jesus Christ is the Son of God.
1. A. T. Robertson, Word Pictures in the New Testament (Baker Books, Grand Rapids, 1933, reprinted 1960, Vol. 6), pp. 240 and 241).
2. Henry Alford, The Greek Testament, (Moody Press, Chicago, 1968, Vol. 4), p. 503.