http://bible.org/article/non-pauline-epistles
The book of 2 John:
7 For many deceivers have gone out into the world, people who do not confess Jesus as Christ coming in the flesh.
This seems to be the big controversy of the time, as John writes about it several times. The warnings against false prophets and false teachers is especially important because the young church is at a growing point here, and if a new (wrong) idea took hold and infected the stalk of the church, so to speak, then everything that grew out of it would be diseased.