Revelation 1:11

11 Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea.

John in the spirit heard a voice that sounded like a trumpet tell him that, "He was the Alpha and Omega" (the First and the Last). He tells John to write what he sees in a book and to send it to seven specific churches in Asia-Minor. He did indeed do that and that is why we have this book today.