https://www.humanreligions.info/1_john_5.html
By Vexen Crabtree 2012
Included as holy:
Protestant Bibles
Title: 1 John
Section: New Testament
Catholic Bible
Title: 1 John
Section: New Testament
Eastern Orthodox Bibles
Title: 1 John
Section: New Testament
Rejected by:
Jewish Tanakh
1 John talks about what the correct beliefs of Christians are. It is written anonymously, but was given the title of "John" because it accepts the theology of the Gospel of John. Not a lot of ancient Bible manuscripts included 1 John in their copies. Dr Thomas Holland notes that of a selection of 400 ancient Greek New Testament manuscripts used by the Nestle-Aland Text (out of 6,000), only 100 contain the collection of Epistles known as the Catholic Epistles. Of these 100, only some include 1 John1. 1 John was either written quite removed from primary sources (hence its adoption of features of the Gospel of John, itself written quite late), or, considered unlikely to be authentic, or, only accepted for a long while by a minority of Christians who believed in particular things about the nature of God and Jesus.
One the more simple and non-theological teachings of 1 John is that Christians must love each other, and must hate darkness and sin.
Vexen Crabtree: About Jesus, the Trinity, the Holy Spirit and Intolerance of those with differing beliefs
1 John 5:7-8 Expanded version (KJV):
7For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
1 John 5:7-8 Original version (NIV):
7For there are three that testify: 8the Spirit, the water and the blood; and the three are in agreement.
1 John 5:7 is no longer included in modern Bibles (for example the English Standard Version and New American Standard Bible), and it is universally acknowledged to be an improper addition to the original text of 1 John chapter 5, except by some Christians. No Greek manuscript before the 9th century includes this text, and of those that do, half of them have it only as a marginal sidenote. When early Christians formulated the idea of the Trinity and debated it, such as Augustine, they quoted 1 John 5 without including the expanded text. It appears in Latin texts, especially those authored in Spain or in places influenced by Spain. It became codified and well known when it appeared in the King James Version.
How did this verse come to be in the Bible?
It could have been done by copyists who thought that the note was in the margin because a previous copyist had missed it but realized his error, and wrote it in the margin. When later copyists saw this, they moved it back into the text, not knowing they were adding a personal note into the text of the Bible.
Or it was accidentally moved into the text of the Bible during a session where multiple copyists wrote whilst the Bible was read out aloud. The oralist has read out the marginal note, and the copyists duly wrote it, not knowing it was set apart from the text.
There have been frequent debates in Christendom about the nature of Jesus (to what extent should he be considered human or divine? Is he supposed to be an eternal being, or was he temporally created?). Using typical John language (i.e. the Greek word logos), the extra phrase of 1 John 5:7 was intentionally inserted in order to strengthen the position of Trinitarians in their debates against other Christians - a ploy which, over time, proved very successful).
Some still argue that 1 John 5:7 is authentic; some argue that particular (and complicated) Greek rules of grammar mean that 1 John 5:7-8 makes more grammatical sense with the extra verse than without it2, or that the inserted verse is so important to the meaning of 1 John 5 that it cannot be omitted. These arguments allow a prediction to made: Those Greek manuscripts that lack 1 John 5:7 must be deficient. However the vast majority of Greek authors did not have it, and, there are no Bible commentaries against these versions complaining of their poor grammar or lack of meaning. In other words, 1 John 5:7 was not noticeable by its absence.
“In the revised versions it is omitted, because it seems quite certain that it was not in the original writing [...] While it appears in most of the best manuscripts which were available for the King James translators, earlier manuscripts found since that time have shown that it was formerly written at the side as a gloss, and was by some transcriber set over in the text itself.”
"The Grestest English Classic: A Study of the King James Version of the Bible and its Influence on Life and Literature" by Cleland Boyd Mcafee, D.D. (1912)3