The eighteenth of the Epistles, and the twenty-third book in the New Testament, is I John. It is the first of three letters written by the apostle to be found in the Bible.
The letter was written around 90 AD, some 50 or 60 years since Jesus' crucifixion, and John was now an old man. He was probably the only one of Jesus' 12 disciples still alive at this time, and so his words would have been well respected amongst Christians.
Like II Peter, written some 20 years before, I John is concerned with false teachers who have entered the church. John tells his readers what it means to have fellowship with God, and he warns them about teachers who reject Christ's birth.