Is it not true that one of the best New Testament proofs for the doctrine of a triune god is found in 1 John 5:7-8? Find out the answer to this question in this article by Gerald Sigal.
It depends on which translation of the New Testament you are using. Many modern translations do not include this supposed proof of a trinitarian godhead. As rendered in the King James Version of the Bible, it reads: "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 witness in earth, the spirit, and the water, and the blood: and these three agree in one." However, these verses do not occur in any reliable Greek manuscript. There is an interesting footnote to the above to be found in the Catholic Jerusalem Bible (1966), which does not have the added words in the main text. It states:
Vulg[ate] vv.7-8 read as followers "There are three witnesses in heaven: the Father the Word and the Spirit, and these three are one; there are three witnesses on earth: the Spirit the water and the blood." The words in italics (not in any of the early Greek MSS, or any of the early translations, or in the best MSS of the Vulg. itself) are probably a gloss that has crept into the text.
These spurious words may have been the work of an overzealous copyist, who inserted this statement so as to lend credence to the doctrine of the Trinity. Whatever its source, the crucial passage is of much later origin than the original authorship of 1 John.