The Comma of 1 John 5:7
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.

It seems like a fairly solid testimony to the Trinity, but is it a genuine article? We've been asked to look briefly at some arguments "for and against" authenticity of this verse (called the Johnanine Comma), and here is what we have found:

External evidence. On these grounds the deck is heavily stacked against the Comma [Marshall, 1 John commentary, 236; Brown, 1 John commentary, 775ff]:

This is fairly powerful evidence, such that would be conclusive for a textual critic. On the other hand, I have been passed these arguments in favor of the Comma:

In sum, while some of the positive evidence is useful (especially the last), the negative textual evidence is strongly against the originality of the Comma.