So my advice is to learn about manuscripts.
Now to a major test:
1 John 5:7-8
1Jn 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
1Jn 5:8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
The NIV and NWT follow the same spirit:
https://www.biblegateway.com/passage/?search=1+John+5:7&version=NIV
https://www.jw.org/en/library/bible/study-bible/books/1-john/5/
The words do not occur in the following corrupted texts:
Aleph (Sinaiticus), A (Alexandrinus), B (Vaticanus), K, L, P, Psi, 048, 049, 056, 0142, MAJORITY, Vulgate-pt, Syriac: Pes.hitta, Harclean, Coptic, (Sahidic), Bohairic, Armenian, Ethiopic (see also "S A B K P Psi 048 33 81 104 614 630 945 1241 1739 1881 2495 Byz Lect one lat earlier vg syr(p,h) cop" -
http://web.ovu.edu/terry/tc/lay281jn.htm )
Here is the vast extant literature which has the text in it:
"... Cursives: pc (9 cited in our other Digest)
Old Latin: c, dem, div, l, m, p, q, r, Vulgate
... the loose ends will not join up grammatically. ... see KJVMT p 115. ..." - A Closer Look: Early Manuscripts & The A.V.; by Jack Moorman, pages 80
Additionally:
"... CYPRIAN, Treatises (I 5:423), "and again it is written of the Father, and of the Son, and of the Holy Spirit, 'And these three are one'" ..." - Early Church Fathers And The Authorized Version, by Jack Moorman, page 60
Additionally:
Additionally:
"... it is found in Codex 61 of the 15-16th century, kept in Dublin and known as the Montfort manuscript, Codex Ravianus (Wizanburgensis) of the 8th century and in the margins of 88 and 629.
The main authorities for the passage are the Old Latin text of the 2nd century, including manuscript r (5/6th cent.) and the "Speculum," a treatise containing the Old Latin text, and several fathers. Fuller (4) p 213, citing Wilkinson, states that the passage was found in the Old Latin Bibles of the Waldenses, whose text pre-dated Jerome's Vulgate. See also Ray (15) p 98, who states that this "Italic" Bible dates from 157 AD. The Old Latin text carried sufficient weight to influence the later copies of the Vulgate, most of which from 800 AD onward incorporated the passage.
The fathers who cite the passage are Tertullian (2nd cent.), Cyprian (250 AD), Priscillian (385 AD), Idacius Clatus (385 AD), several African writers of the 5th century and Cassiodorus (480-570 AD).
The combined influence of these authorities, together with grammatical difficulties which arise if the Comma is omitted, was sufficient to ensure its place in most editions of the Textus Receptus-see Berry's text- where it undoubtedly belongs.
See Hills (3) p 209, (38) p 210, the TBS (58) "Notes on the Vindication of I John 5:7" and Ruckman (2) p 128-9, (31) p 334. The omission of the Comma from the majority of the manuscripts most likely stems from the influence of Origen and some of his supporters, who did not accept the doctrine of the Trinity. This text is also discussed at length in Part 3. ..." -
http://ecclesia.org/truth/manuscript_evidence.html
Additionally:
"... Here is just a partial list of those who contended for the authenticity of this verse.
Cyprian - 250 AD, Athanasius 350 A.D., Priscillian -385 AD, Jerome 420 AD, Fulgentius (late 5th century), Cassiodorus, Isidore of Seville, Jaqub of Edessa, Thomas Aquinas, John Wycliffe, Desiderus Erasmus, Stephanus, Lopez de Zuniga, John Calvin, Theodore Beza, Cipriano de Valera, John Owen, Francis Turretin, John Wesley, John Gill, Matthew Henry, Andrew Fuller, Luis Gaussen, Frederick Nolan, Robert L. Dabney, Thomas Strouse, Floyd Jones, Peter Ruckman, George Ricker Berry, Edward F. Hills, David Otis Fuller, Thomas Holland, Michael Maynard and Donald A. Waite. ...
... It should also be noted that Michael Maynard significantly points out that there are only 5 remaining Greek manuscripts that even contain the epistle of 1 John in whole or in part that date from the 7th century or before. That is a whole lot of time to have past by with only 5 partial Greek witnesses that remain today that were written within the first 700 years of Christianity.
And among these 5 early manuscripts only 2 of them agree with each other in 1 John 5:6-8. Sinaiticus does not agree with Vaticanus, or Alexandrinus or with 0296. Sinaiticus and A both say "by water and blood AND SPIRIT" in verse 6 instead of "by water and by blood". Then Alexandrinus "not by water only but by water AND THE SPIRIT" instead of "not by water only, but by water and the blood" and 0296 omits the verb "are" (εισιν) in verse 7 and has the unique word order of "by water AND SPIRIT and blood" in verse six. ...
... It is found in several Greek texts - Erasmus, Stephanus, Beza, Elziever, Scrivener and Modern Greek Bible; it is quoted by several church fathers as Cyprian 250 AD, Athanasius 350 A.D., Priscillian -380 AD, Varimadum 380 A.D., Jerome 420 AD, Victor Vitensis 430 A.D., Fulgentius (late 5th century), Cassiodorus 580 A.D, and is found in many ancient versions of the Bible including the Old Latin, and is found in some copies of the Syriac, Armenian, Georgian and Slavonic ancient versions.
... 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).
It was part of the text of the Old Latin Bible that was translated in the second century, as it witnessed by a remaining copy that we have today. It is found in "r", a 5th century Old Latin manuscript.
JEROME tells us that certain Arian scribes were removing this section of Scripture from the Greek manuscripts.
Even more to the point is the testimony of Jerome on this matter. Jerome was commissioned by Damasus, the bishop of Rome, to prepare a standard Latin translation of the Holy Scriptures to replace the former Latin translations which had grown in multiplicity by the late 4th century. Jerome did this, utilizing the Greek as his source for revision of the Latin New Testament for his Vulgate.14 At one point in his work, JEROME NOTED THAT THE TRINITARIAN READING OF I John 5:7 WAS BEING REMOVED FROM GREEK MANUSCRIPTS WHICH HE HAD COME ACROSS, a point which he specifically mentions. Speaking of the testimony of these verses he writes,
"Just as these are properly understood and so translated faithfully by interpreters into Latin without leaving ambiguity for the readers nor [allowing] the variety of genres to conflict, especially in that text where we read the unity of the trinity is placed in the first letter of John, where MUCH ERROR HAS OCCURRED AT THE HANDS OF UNFAITHFUL TRANSLATORS CONTRARY TO THE TRUTH OF FAITH, WHO HAVE KEPT JUST THE THREE WORDS WATER, BLOOD AND SPIRIT IN THIS EDITON OMITTING MENTION OF FATHER, WORD AND SPIRIT in which especially the catholic faith is strengthened and the unity of substance of Father, Son and Holy Spirit is attested."
“Thus, we see that JEROME SPECIFICALLY MENTIONED THAT THIS VERSE WAS BEING REMOVED FROM GREEK MANUSCRIPTS IN HIS DAY. Logically, we can suppose that for him to recognize the absence of this verse as an omission from the Greek texts, he must have been aware of Greek manuscripts which contained the Comma in the time of his preparation of the Vulgate for the general epistles (395-400 AD), a time much earlier than is suggested by the dating of currently known Comma-containing Greek mss.” ..." -
https://brandplucked.webs.com/1john57.htm
More in a moment.