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 3:13-16
"Marvel not, my brethren, if the world hate you. We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death. Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him. Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren"