Why is 1 John 5:7–8 omitted from almost all all modern translations of the Bible? Is it because the doctrine of the trinity is an invention of the modern church, inserted hundreds of years after the death of the author.

 

In the earliest manuscripts, 1 John 5:7–8 reads:

For there are three that bear record: the Spirit, and the Water, and the Blood, and these three agree in one.

This passage was edited in a Latin manuscript around the end of the 4th century, to read:

For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witnesse in earth, the Spirit, and the Water, and the Blood, and these three agree in one.

Sorry to disappoint, but there is no conspiracy here. The discrepancies have been under scrutiny — out in the open — since at least the 16th century. Even Isaac Newton wrote about it. There is an excellent summary at Johannine Comma - Wikipedia.

Though the KJV included the addition, other early vernacular translations didn't. And this makes sense given the manuscripts available at that time.

Scholars now have many more manuscripts, and have largely corrected the error. The translation I read, the New International Version, leaves out the added material, but explicitly mentions it in a footnote.

We can speculate about why the change was made, and it seems reasonable to suggest that it was to more thoroughly support the doctrine of the Trinity. However, the original unedited text of 1 John 5:5–7 already speaks clearly about two of the persons of the Trinity:

This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. For there are three that testify: the Spirit, the water and the blood; and the three are in agreement.

And verse 8 completes the Trinity with the Father (who is undoubtedly assumed in 5–7):

For there are three that bear record: the Spirit, and the Water, and the Blood, and these three agree in one.