The command to John in Revelation 1, verse nineteen gives the key to the book of Revelation. Understanding revelation is dependent upon using the key.
Understanding Revelation Commentary 7
Rev. 4:1 After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter.
The Trumpet
This would be the 1Thess. 4:16 and 1 Cor. 15:52 trumpet. I will show you things which must be hereafter means after chapters 2 and 3. After the church is gone. Remember the key back in Revelation chapter 1? This is not the Jewish feast of trumpets, Yom Teruah (Rosh Hashanah) trumpet. The feast of trumpets will be realized at the end of the tribulation period. Just before Jesus returns in the clouds with power and great glory. This is not the seventh trumpet of Revelation 11:15. That trumpet sounds in the middle of death and destruction upon the earth.
This is the rapture trumpet. Come up here! The church is no longer present on the earth after chapter 4. We don’t see the church again until Rev 19:7-19:14. They are riding white horses with the armies of heaven. Following Him who is clothed with a vesture dipped in blood. His name is called The Word of God! (This is Jesus).
2 And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne.
3 And he that sat was to look upon like a jasper and a sardine stone: and there was a rainbow round about the throne, in sight like unto an emerald.

And he that sat was to look upon like a jasper and a sardine stone:

and there was a rainbow round about the throne, in sight like unto an emerald.
The 24 Elders
4 And round about the throne were four and twenty seats: and upon the seats I saw four and twenty elders sitting, clothed in white raiment; and they had on their heads crowns of gold.
These elders are probably representative of the church. It could be Israel and the church. Or it could be angelic beings. We do know that the saints are clothed in white raiment. Rev. 19:8 And to her was granted that she should be arrayed in fine linen, clean and white: for the fine linen is the righteousness of saints. Angels are also clothed in white. Matt. 28:3 His countenance was like lightning, and his raiment white as snow. Representatives of the church makes sense though. The church has been removed from the earth and is now with Jesus in the heavenly realm. Notes
5 And out of the throne proceeded lightnings and thundering and voices: and there were seven lamps of fire burning before the throne, which are the seven Spirits of God.
Seven being the complete spirit of God.
The Four Beasts
6 And before the throne there was a sea of glass like unto crystal: and in the midst of the throne, and round about the throne, were four beasts full of eyes before and behind.
7 And the first beast was like a lion, and the second beast like a calf, and the third beast had a face as a man, and the fourth beast was like a flying eagle.
8 And the four beasts had each of them six wings about him; and they were full of eyes within: and they rest not day and night, saying, Holy, holy, holy, Lord God Almighty, which was, and is, and is to come.
These creatures are angelic beings engaged in worship. We get a picture of these creatures also in Ezekiel 1:4-7, 10.
9 And when those beasts give glory and honour and thanks to him that sat on the throne, who liveth for ever and ever,
10 The four and twenty elders fall down before him that sat on the throne, and worship him that liveth for ever and ever, and cast their crowns before the throne, saying,
11 Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.
