The portion of 1 John 5:7 reading "the Father, the Word, and the Holy Ghost: and these three are one" in the KJV is only to be found in the Old Latin and Vulgate versions of the bible, it occurs in none of the Greek manuscripts, early or late, major or minor. As such it has almost no attestation and is almost certainly not part of the original autograph. Personally while my daily reader is the NKJV I don't use the end of 1 John 5:7 in witnessing to JWs, especially because this is one of the weak verses they've been trained on.

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Pastor Andy Webb
Providence PCA, Fayetteville, NC
BUILDING OLD SCHOOL CHURCHES
"Providence is a Christian's diary, but not his Bible. Sometimes a bad cause prevails and gets ground; but it is not to be liked because it prevails. We must not think the better of what is sinful, because it is successful. This is no rule for our actions to be directed by." - Thomas Watson