according to the book i am reading, 1st john 5:7,8 is the STRONGEST argument for the trinity in the king james bible. it states clearly that there are 3 that bear witness in heaven: the Father, the Word, and the Holy Ghost. john has also stated in the gospel that he wrote, that the word became flesh.

it has been changed in ALL other bible versions.

(Jehovah's Witnesses do not believe the trinity, or the Godhead exists, and don't believe that Jesus Christ is the Son of God; without this verse, it is much more difficult to convince them that the trinity does exist)

there haven't been many greek manuscripts which contain 1st john 5:7,

HOWEVER, the evidence for it is still very strong: there have been numerous manuscripts in other languages which contain it, and many church fathers back then quoted it; it was just not found in a lot of manuscipts; what gives?

two possible reasons:
1)scribal error: the three that bear witness in verse 7 was confused to be that of verse 8, hence the accidental skipping of a line
2)numerous cults believed that God was all-inclusive(or something like that where the entire world make up God)

i'd post more, but library's going to close; this should give you something to chew on for a bit.