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.

"After this..."  refers to events after what happened in the first three chapters.

A door into heaven is opened for John to see inside.

Does he actually see into heaven or is he shown a representation of what heaven will be like?

The Bible does say that no man has ever seen God (John 1:18; 1 Tim. 6:16).

What John sees is a vision in which symbols are pictures of ideas.

The voice like a trumpet is probably the same voice as in 1:10 which is probably Christ 1:18.

John is going to be shown things that will come to pass.

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.

He is brought before the throne of God.

God is not described as a man since He is not a man (Jn. 4:24).

The one who sat on the throne was like jasper.  Probably a diamond (21:11).  Descriptive of
God's holiness and righteousness and purity.

Sardius is a stone that is thought to have been fiery red.  Hence would represent God's wrath and
judgment.

A rainbow was around the throne like emerald..  This could be describing the mercy of God.

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.

Around the throne were 24 seats or thrones.

24 is two twelves.  Which probably represents the twelve tribes of Israel (the Old Covenant) and
the twelve apostles (the New Covenant).  Thus representing the redeemed of both covenants
(Heb. 9:15; Eph. 2:16).

They were clothed in white representing purity and holiness (Rev. 3:5).

They were wearing golden crowns.  These are victory crowns (2:10).

5 And out of the throne proceeded lightnings and thunderings and voices: and there were
seven lamps of fire burning before the throne, which are the seven Spirits of God.

"Lightnings and thunderings and voices" represent the presence of God.  Show that He is active.

This reminds one of when God descended upon Mt. Sinai (Ex. 19:16-19).

Seven lamps of fire.  Probably the complete Spirit of God which does His will.  See Rev. 1:4.

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.

Before the throne is a sea of glass like unto crystal.  The explanations of this are varied.

The word "sea" is often used to describe the masses.  Being like glass could represent the
calmness that comes from God.

Some even say that this sea of glass represents the providence of God.  However, this seems like
a stretch.

"Beasts" is better interpreted as "living creatures."

The four faces could represent four qualities of God and His angels.
     Lion = Strength.
     Calf (ox) = service.
     Man = Intelligence above all creatures.
     Eagle = swiftness.

Being full of eyes represents their being able to see all things.  Nothing escapes their gaze.

They never cease to praise God.

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.

When the creatures worship, the 24 elders follow suit.

They realize that God is the source and cause of their salvation.

Throwing their crowns before His feet shows their submission and gratitude.