Modern Bibles deleted 1 John 5:7's Trinity language

The extra phrase in 1 John 5:7 ("the Father, the Word, and the Holy Ghost: and these three are one") is called the Comma Johanneum, and it does not show up in any Greek manuscript until the 15th century. When Erasmus produced his first two printed Greek New Testaments in the early 1500s, he left it out and said plainly that he couldn't find it in any Greek manuscript, because it wasn't there. It had entered the Latin tradition as a marginal gloss that eventually got copied into the text of the Vulgate. His critics pressured him to include it, and it's almost unanimously agreed among textual scholars that someone then produced a Greek manuscript containing the phrase specifically to meet his challenge. Erasmus, having said he would include it if a Greek manuscript could be produced, kept his word in his third edition. From there it flowed into the Stephanus and Beza editions, into the printed Greek text called the Textus Receptus, and into the King James Version.

The strongest evidence that it's not original isn't just its absence from early manuscripts: it's its absence from the fight where it would have mattered most. The fourth century saw ferocious, extended debates over the Trinity, especially between the orthodox side and Arius at the Council of Nicaea. If 1 John 5:7 had read this way, it would have been the single most obvious proof-text for the doctrine, and someone would have used it. Nobody ever does. That silence, combined with zero Greek manuscript support before the 1400s, is decisive: the doctrine of the Trinity doesn't rest on this verse and never did, but the phrase itself is a documented late addition, not a modern deletion.