The Father, the Word, and the Holy Ghost: and these three are one

[tonychanyt]

KJV, 1 John 5:

7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.

There is some controversy about this verse.

Wiki:

The Johannine Comma (Latin: Comma Johanneum) is an interpolated phrase (comma) in verses 5:7–8 of the First Epistle of John.[1]
The text (with the comma in italics and enclosed by square brackets) in the King James Bible reads:
7For there are three that beare record [in heaven, the Father, the Word, and the Holy Ghost: and these three are one.] 8[And there are three that beare witnesse in earth], the Spirit, and the Water, and the Blood, and these three agree in one.
— King James Version (1611)
The passage appears to have originated as a gloss in a Latin manuscript around the end of the 4th century,[3]
The comma is absent from the Ethiopic, Aramaic, Syriac, Slavic, Armenian, Georgian, and Arabic translations of the Greek New Testament.
The earliest known Greek ms. occurrence appears to be a later addition to a 10th-century manuscript now in the Bodleian Library. The exact date of the addition is not known; in this manuscript, the Comma is a variant reading offered as an alternative to the main text.

Without the comma/addition, it reads:

7 For there are three that testify: 8 the Spirit and the water and the blood; and these three agree.

There are good reasons to believe that the Johannine Comma was inserted:

1. The words did not appear in any Greek manuscript before the 14th century CE.
2. They were absent from all ancient translations of the Bible.
3. No Church Father quoted them in discussions about the Trinity.

How do we know that manuscripts used to compile the Bible or the Bible itself are not forged or changed?

This is where textual criticism comes in, which is a scholarly discipline that aims to reconstruct the original text of a document, particularly ancient texts, when the original is no longer available. 1Jn 5:7 is an example of this kind of criticism.