Is 1 John 5:7-8 (the Comma Johanneum) a Latin corruption? Is the biblical passage at 1 John 5:7-8 (the Comma Johanneum) a Latin corruption that has entered the Greek manuscript tradition?
The authenticity of the Johannine comma, found in 1 John 5:7, has been a subject of debate from the early sixteenth century. Wikipedia says the general consensus today is that the passage is a Latin corruption that entered the Greek manuscript tradition in subsequent copies. By the way, the Johannine Comma is not to be confused with the modern punctuation mark (,); rather, the word comma in this sense came to Latin via the Greek word komma, which meant a clause, or a short grouping of words.

The text including the comma (here, in brackets) reads:


  
  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).
  


If the comma is removed, it would read:


  For there are three that bear record, the spirit, and the water, and the blood: and these three agree in one


The editors of the 1808 New Testament in an improved version found the following reasons for rejecting the comma:


This text concerning the heavenly witnesses is not contained in any Greek manuscript which was written earlier than the fifteenth century.
Nor in any Latin manuscript earlier than the ninth century.
It is not found in any of the ancient versions.
It is not cited by any of the Greek ecclesiastical writers, though to prove the doctrine of the Trinity they have cited the words both before and after this text
It is not cited by any of the early Latin fathers, even when the subjects upon which they treat would naturally have led them to appeal to its authority.
It is first cited by Virgilius Tapsensis, a Latin writer of no credit, in the latter end of the fifth century, and by him it is suspected to have been forged.
It has been omitted as spurious in many editions of the New Testament since the Reformation:—in the two first of Erasmus, in those of Aldus, Colinaus, Zwinglius, and lately of Griesbach.
It was omitted by Luther in his German version.


As this site notes, the Johannine comma has been ommitted from the United Bible Societies Version 4 (UBS4), widely accepted as representing current scholarly opinion.



Overall, I believe it would be safe to say that 1 John 5:7-8 is a later insertion, and that, as it first appears in the Latin milieu, the evidence strongly supports it being a Latin corruption that was copied back into later Greek manuscripts.