I and others believe that it is sort of added, that it is added by some ignoramus. We do not want, however, to translate it because of the word "testimony," because in heaven there will be no need for a testimony as it is written: "we will see God face to face. " There, the Trinity will declare Himself.
. . .But this verse seems to have been inserted by the Catholics because of the Arians, yet not aptly."
"For there are three." This locus the Greek codices do not have. It seems that it was inserted ineptly by the eagerness of ancient theologians against Arius, if one looks at the analogy of faith. Where God is seen, there is no need for a testimony, but here it is needed, here we have it in the word, and we do not want to have it any other way, since there is no testimony in heaven and no faith, which are of this life. Therefore, we leave out this text. Also the subsequent text ridicules this verse. And I can make fun of it easily because there is no more inept locus for the Trinity.
(Culled from Luthers Table Talks and his lectures on Johns first Epistle.)







