If you do not know what the Johannine Comma is, I will explain it to you (hint: it is not John’s punctuation). The best way to understand it is to compare 1 John in the KJV and a modern translation.
“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.” (1 John 5:7–8 KJV)
“For there are three that testify: the Spirit and the water and the blood; and these three agree.” (1 John 5:7–8 ESV)
What we see is that the King James records a comment about the Trinity while all modern translations are missing those words. Textual criticism has demonstrated that the King James version of those verses are not original and they were added much later on. I will not go into detail about this but you can find more information here.
I am more interested in the ramifications of this being a later addition. I have heard skeptics suggest that this is proof that the Trinity is a late invention. Should this issue cause us to doubt the doctrine of the Trinity?
One of the things that we should note is that the church fathers were talking about the Trinity long before the Johannine Comma came on the scene. No one waited until this verse appeared to discuss the Trinity.
We should also realize that there is plenty of biblical evidence for the Trinity. In another post, I demonstrated some of the places (and this is not an exhaustive list) in the New Testament that we see clear evidence of the Trinity.
Would we like the Johannine Comma to be original? Of course we would. But even if it was original, it would not be the deciding factor for the Trinity. It simply would have been one verse among many verses that point toward the Trinity.