"For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." (I John 5:7)
This is the famous Trinitarian proof text which F.J.A. Hort hoped to remove from the New Greek Text (see Life and Letters of Fenton John Anthony Hort).
It is missing from the New Greek New Testament Text, the English Revised Version and most modern translations. Yet, this scripture is in thousands of ancient manuscripts. It appears in the oldest Latin manuscripts dating to 150 AD., and is quoted by early church fathers. Hence, it is impossible for it to be a "late addition"