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.
I John 4:1-3
"Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: and every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world"