Good morning OneGodBeliever;
You ask a good question. The verse you refer to is part of the Comma Johanneum.
1 John 5:6-9 (KJV)
6 This is he that came by water and blood, [even] Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.
9 If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
1 John 5:6-9 (RSV)
6 This is he who came by water and blood, Jesus Christ, not with the water only but with the water and the blood.
7 And the Spirit is the witness, because the Spirit is the truth.
8 There are three witnesses, the Spirit, the water, and the blood; and these three agree
9 If we receive the testimony of men, the testimony of God is greater; for this is the testimony of God that he has borne witness to his Son.
There is debate as to why verse 7 in the KJV version appeared in the form it does as it does not appear in most other translations. Here apparently is the answer:
This verse has virtually no support among the early Greek manuscripts . . . Its appearance in late Greek manuscripts is based on the fact that Erasmus was placed under ecclesiastical (religious) pressure to include it in his Greek NT of 1522, having omitted it in his two earlier editions of 1516 and 1519 because he could not find any Greek manuscripts which contained it.
Words and phrases are sometimes changed to be more specific. And there are some other areas of the New Testament which are similar. The concept of the Trinity is not solely based on 1 John 5:7. Rather, it is a realization of Scripture which leads one to consider that Father, Son, and Holy Spirit have a common relationship. Why did the authors of the King James translate 1 John 5:7 the way they did is beyond me. But, it does not contradict the intent of the New Testament overall. Instead, it defines their relationship in a single sentence. Cheers, John