4:1 After these things I looked, and behold, a door standing open in heaven, and the first voice which I had heard, like the sound of a trumpet speaking with me, said, "Come up here, and I will show you what must take place after these things."
After these things I looked, and BEHOLD,
The "things" John is referring to are:
Now the "churches" are not referred to again in Revelation until the postamble. The view is switched from being on the earth to being in heaven in this chapter. Heaven is not familiar to John, and thus his amazement is expressed in "BEHOLD". All the things, symbols, beasts, phenomena (heavenly and earthly) that he will write about are just plainly outside the common person's experience. John is given a view which is not restricted by the physical dimensions of space and time.
A DOOR standing open in HEAVEN, and
The doors in the "churches" were 'an open door' promised to Philadelphia (Revelation 3:8) and 'a closed door' in Laodicea (Revelation 3:20).
the FIRST VOICE which I had heard, like the sound of a trumpet speaking with me, said,
John had heard this voice before (Revelation 1:10) when he was given the command 'to write' this Revelation. Communications in the spiritual realm is not like that on earth. When we are taken into heaven as recorded in 1 Thessalonians 4:16-17, there is a shout, a voice, and the trumpet of God to cause the event. These combined will be a tremendous burst of energy powerful enough to raise the dead and those of us who are ALIVE to be gathered together to join Christ. Furthermore, in 1 Corinthians 15:50-58, the energy causes a transformation from fleshly (perishable) to imperishable. IT IS POWERFUL!
COME UP HERE, and I will show you what MUST take place after these things.
"HERE" is where Christ is (Acts 1:1-11), and that is heaven. Jesus told us of heaven, His home, in John 14:1-4. He also told us that we would not be left alone here on earth but that we would have the Spirit (Comforter) between now and when we "GO UP THERE".
That "FIRST VOICE, Jesus Christ", will show John the future. We may desire that some of the items being described in the rest of Revelation are optional, but they are not; they MUST take place.