Based on not one actual God breathed scripture (so the unitarians would say). Also, the concept that it is only a descriptive term is closer to modalism, not to orthodox trinitarianism (whether RCC, EOx or Protestant. Or Nestorian, Coptic, Jacobite or several other now-minor variants of trinitarianism for that matter.)
I think you meant 1 John 5:8, by the way. And the second half of that is only found in super-late manuscripts. (The first half says there are three that bear witness: the Spirit and the water and the blood. It’s the second half that has become notorious.) The Eastern Orthodox are extremely much in favor of the Trinity, and were part of the Church who “eventually agreed on the doctrine of the Trinity” “after many debates”. Just like the RCCs, they would have no reason at all to remove it from Bibles. But trinitarians of all shades would have plenty of reason to add it to Bibles. Which happens to be how the textual evidence comes out.
The EOx weren’t the only ones combatting that heresy, and indeed as a heresy it far precedes the split between the RCCs and the EOx. Do you even know what the heresy of Sabellianism is?! “Trinity is just a descriptive word for The Father, The Son, and the Holy Spirit.” You or your source actually affirmed it by accident at the start of your reply.
Sabellius was a bishop in the precinct of Arius, back in the late 200s or early 300s, who was preaching something Arius understood to be the denial of the distinction of the Persons, including of the Father and the Son. In responding against this preaching–which Sab might not have intended but was only being inept about–Arius went to the other extreme and insisted upon the total distinction of the Persons of the Father and the Son as categorically different entities, the Father uncreated but the Son created. (Arius was also zealous to deny cosmological tri-theism, which he thought the orthodox party was promoting.)
The bishops hashed this out in the early 300s; where, had the Trinitarian Comma been known about, it would have been deployed against both Arius and Sabellius. The split between the RCCs and the EOx happened no earlier than the 600s, after they had solidly affirmed the Two Natures doctrine of Christ over against the One Nature trinitarian advocates (roughly speaking the Coptic and Ethiopian churches), partially on the charge that the ON advocates were trending into Sabellianism. Which by the way wasn’t true, but it made for good rhetorical ammo. (On the contrary, the first textual indications of the Trinitarian Comma itself come from a Latin text in the late 300s to either the heretic Spanish Priscillian or to his disciple Bishop Instantius. But throughout the 400s the gloss was quoted by Latin Fathers in North Africa, i.e. the same region as the Coptic and Ethiopian churches. Also in Italy.)
The term only starts showing up in Old Latin and Vulgate texts of the actual scriptures (not references from Patristic commentary) in the 500s; where it differs substantially in wording from text to text (or family to family). Meaning they weren’t copying it from a prior text until it was already established in the text as ‘text’. It shows up in absolutely no other early languages, aside from a few relatively late Greek texts where usually it is treated as a marginal comment (apparently copied from the Latin).
The Greeks didn’t (usually) include it, because they realized the text was spurious and had crept into the actual text over time in Latin (nowhere else). Neither the Eastern Orthodox (Greek or otherwise), nor any other trinitarian, would have had reason to erase it, including in dispute against the occasional Sabellianist. They would have been deploying it against Sabellians instead, or at least trying to defend its use to Sabellians.
(Similarly, when Arius and those like him tried to make a point from the way GosJohn 1:3-4 was used before the Nicean Council, the text wasn’t deleted or even altered. The only thing that was changed was what we would call the punctuation, so that “In Him was life” would start a new sentence instead of finishing up one.)
Your source is quite wrong on almost all points. I recommend getting a new source.
Not just one, but hundreds. Just like trinitarian doctrine is not based on just one (even the Trinitarian Comma, thank God!) but hundreds.
You may not agree with how it is based on hundreds of scripture, but that doesn’t change the fact that I can and do derive it from hundreds of scripture. You can continue to ignore that I do so from hundreds of scripture references, but that doesn’t change the fact that I do so. Squinting shut your eyes and plugging up your ears, doesn’t mean the light does not exist.