The author identifies himself as John, historically considered to be the Apostle John, who also wrote the Gospel of John along with II John, III John and the Book of Revelation. Liberal theologians disagree, conceding only that all three epistles were likely written by the same person.
The theme of the book is love and fellowship with God. This is demonstrated in part by one of its more well-known verses, 1:9 ("If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.")
I John also contains one of the more controversial verses (in terms of whether it is in the original or not): 5:7, commonly known as the Comma Johanneum ("For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.") The verse is considered to explicitly support the doctrine of the Trinity; it is included in the King James but excluded from some modern versions (other versions include it but with footnotes that it is not found in most original documents). Modern scholarship has questioned whether it was in the originals or if it was a later addition; adherents of the KJV Only movement often make this a point in claims that modern translations are "corrupt". David Cloud of Way of Life Literature (a KJV Only advocate, though not to the radical extreme in his views) wrote a defense of the Comma at his website.