"This verse in the KJV is to be rejected (with RSV). It appears in no
ancient Greek MS nor is it cited by any Greek fathers; of all the
versions only the Latin contained it, and even this in none of its most
ancient sources. The earliest MSS of the Vulg. do not have it. As [CH]
Dodd (Johannine Epistles, p. 127n) reminds us, "It is first quoted as a
part of 1 John by Priscillian, the Spanish heretic, who died in 385,
and it gradually made its way into MSS of the Latin Vulgate until it
was accepted as part of the authorized Latin text." The mention in the
true text (vs. 8) of the three witnesses which agree naturally led to
an interpretation along trinitarian lines, and this occasioned the
present gloss which appears in various forms in MSS and quotations from
the fifth century onward" (293-294).
One of the translators of the NIV also writes the following about 1
"Anyone who uses a recent scholarly version of the NT will see that
these words on the Trinity are not in verse 7. This is because they
have no basis in the Greek text. Under Roman Catholic pressure, Erasmus
inserted them from the Latin Vulgate. They are not a part of the
inspired Bible" (Ralph Earle, Word Meanings in the NT, p. 452).
Another authority states:
"We need not hesitate to declare our conviction that the disputed words
were not written by St. John: that they were originally brought into
Latin copies in Africa from the margin, where they had been placed as a
pious and orthodox gloss on ver. 8: that from the Latin they crept into
two or three late Greek codices, and thence into the printed Greek
text, a place to which they had no rightful claim" (A Plain
Introduction to the Criticism of the NT. Cambridge: 1883, 3rd Ed. P.
654. Comments made by FHA Scrivener).
Robert M. Grant makes this comment about 1 John 5:6-8:
"To this mysterious but not theologically useful passage a Spanish
Pricillianist in the late fourth century added explicitly trinitarian
language so that it would mention three witnesses 'on earth' and end
thus: 'And there are three witnesses in heaven, the Father, the Word,
and the Spirit, and these three are one.' The addition is suitable in a
Johannine context, for it refers to the Logos as John does and is
ultimately based on 'I and the Father are one' (John 10:30).
Unfortunately it is not genuine, since it appears in no old manuscript
or versions or in any early [church] fathers" (Gods and the One God, p. 151).
I would also advise you to read William Barclay's commentary on 1 John and Raymond Brown's extensive treatment of the subject in his Anchor Bible Commentary.