[Ali Imran]

1 John 5:7-8 is not a forgery in the sense of being a deliberate falsification of the original text of the Gospel of John. It is a later addition to the letter of 1 John, included in some manuscripts but omitted by most modern translations because of its lack of support in the older manuscripts....

Ok. We can disagree on the terminology. What is true is that someone added verses to it, a few hundred years after that epistle was written. We can call it a corruption. Bruce Metzger called it a corruption.