I was in the Spirit on the Lord’s Day, and I heard behind me a loud voice, as of a trumpet, saying, “I am the Alpha and the Omega, the First and the Last,” and, “What you see, write in a book and send it to the seven churches which are in Asia: to Ephesus, to Smyrna, to Pergamos, to Thyatira, to Sardis, to Philadelphia, and to Laodicea.” Revelation 1:10-11
When John speaks about being in the Spirit, He refers to receiving the vision of revelation. We must remember that all John wrote in Revelation was based on a vision he received from the Lord. When God gives someone a vision the person feels as if they have stepped into a reality and all they are seeing before them they are seeing with their eyes.
On the Lord's day points to John receiving this on Sunday. Under the Old Covenant the nation of Israel honored the Sabbath which was Saturday for it honored the day the Lord rested during creation. But under the new covenant through Jesus Christ the Lord's day is Sunday for it is the day that Jesus rose from the dead.
The voice that John hears behind him is the voice of the Lord and is loud like a trumpet. When the Lord calls His bride home before His wrath is poured out on the earth will be the last trumpet. And John heat can only compare the intensity and majesty of God's voice to a loud trumpet.
John hears the voice behind Him call Himself the Alpha and Omega, the First and the Last. John knows that this is Jesus Himself speaking to him. Just because a person receives a vision doesn't mean it is from the Lord. Satan through out the history of man have given individuals false visions which have led to incredible deception, which inevitably has led to many of the cults and false religions around the world. John on the island of Patmos as an old man would have had a very difficult time in exile and the Lord made it very clear to him who indeed was speaking to him.
The Lord tells John what you see write in a book. Again John is receiving a physical vision from the Lord playing in front of him like a movie. Much of what John will see will overwhelm him and may even frighten him at points. But in the end John would see the final victory of the Lord over wickedness on the earth.
John again is called to send this to the seven churches. Much of what we find written in the New Testament were original letters that the Holy Spirit inspired men to write to local churches and individuals. God throughout history has sustained His word by His mighty power and grace. Revelation is a book we should diligently study as it will strengthen our faith and increase our understanding of God and His kingdom.