Read 1, 2 and 3 John tonight. I didn't realize it's the same John who wrote the fourth gospel as well as the book of Revelation. The guy was a prolific writer!
Anyway, the three letters are pretty short, and are to Christians warning them about false prophets, which evidently was a big problem after the original apostles were all dead. There were people preaching in the name of Jesus but who were really just trying to spread some other message and using his name to get attention. John says to test if someone's a real preacher or a false prophet (good or evil), there are two things to ask them: the first is, was Jesus a real man, and the answer, of course, should be a resounding "yes!" (because that was kind of the point -- he suffered as a human being suffers through pain and death), he wasn't just an apparition. The other test is to check whether these preachers/prophets follow the 10 commandments and show brotherly love, which is the foundation of all Chrisitianity. If you hate your neighbour you can't possibly love God, John says.
"Beloved, if God so loved us, we ought also to love one another. No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us." -- 1 John 4:11-12