I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty.
Here the Lord is reminding us that He was here when the world was created, and He will be here at the end of the world as we know it. He is the Almighty God over heaven and earth.
 I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ.
John is giving us his testimony here. John was imprisoned on the Isle of Patmos for preaching the Gospel, and it was fairly common in that era for Christians to be persecuted.
 I was in the Spirit on the Lord’s day, and heard behind me a great voice, as of a trumpet,
John is revealing here that the Spirit of the Lord came to him on the Lord’s day (likely the Sabbath, either Saturday or Sunday). When the Spirit moved, he saw a vision. In this vision, he literally heard, in his words, “a great voice, as of a trumpet.”
 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.
God is telling John that He is the beginning and the end, the one and only God. He is telling John to put what he sees in this vision in a book, and send it to the seven major churches of the day. These seven churches have further significance, as we will see later.