Joining in the discussion, although I have a bit above.
The 500 Greek manuscripts give us a good picture that the Greek manuscript line normalized without the verse from c. AD 700 to AD 1200. At that point the Lateran Council began a return of the Greek text which included the Council having the verse in Greek and Latin (discussing the doctrines of Joachim Fiore), Codex Ottobonianus, Manuel Calecas and Joseph Bryennius, before Erasmus and the 1500s activity.
Also we have a good picture of the Latin line from about AD 200 (Tertullian and Cyprian) continuing onward with the verse in an estimated 1,000 manuscripts. This includes Old Latin mss. which line was translated no later than the 2nd century. And heavenly witnesses usage by about 100 Latin authors from the 300s consistently through to the Reformation era.
Then we have cross-language evidences like Jerome's Prologue to the Canonical Epistles that points to ancient mss. with the verse.
We have the Council of Carthage of 484 with 400+ orthodox confirming their faith specifically citing the Johannine heavenly witnesses verse. This shows that the Old Latin line having the verse was widely accepted without any pushback.
Similarly the grammatical solecism without the verse in the short Greek text tells any true Bible believer that this verse is from John, along with powerful corroborating stylistic and internal evidences. A simple example is the "Witness of God" of verse 9 points right back to the heavenly witnesses of verse 7. Another is the wooden redundancy of verse 6 to verse 8 when the heavenly witnesses are removed. Another is the beautiful parallelism, in the minds of the short text afficianados there is not even a reference to heavenly or earthly! This is a beautiful Johannine parallelism, not that of Clunk the Margin Writer and Flunk the Interpolator.
We also have some Greek evidences such as the Disputation of Athanasius against an Arian at Nicea that show the verse. And the Synopsis of Scripture.
So to focus almost obsessively on simply Greek manuscripts, which are almost all quite late, is a trick of the modern textual critics who really do not understand the evidences. For this verse, the Greek manuscripts are simply one modest group of evidences.
It is exceedingly easy to explain the verse dropping out of the Greek line, but a wacky margin insertion that fixes the Greek text solecism in translation from Latin, and develops a beautiful harmonious parallelism is, for the Bible believer, a road too far.
As Demian said:
"Five manuscripts in a period of 700 years is good enough for you to establish a firm tradition of Greek manuscripts from the autographs all the way until the 7th century? Five manuscripts in 700 years!"
Blessings and grace in Jesus name!
Dutchess County, NY USA