Altar
The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.
Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.
Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.
The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.
In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.
Revelation 10
1
I saw a mighty angel coming down out of the sky, clothed with a cloud. A rainbow was on his head. His face was like the sun, and his feet like pillars of fire.
2
He had in his hand a little open book. He set his right foot on the sea, and his left on the land.
3
He cried with a loud voice, as a lion roars. When he cried, the seven thunders uttered their voices.
4
When the seven thunders sounded, I was about to write; but I heard a voice from the sky saying, "Seal up the things which the seven thunders said, and don't write them."
5
The angel who I saw standing on the sea and on the land lifted up his right hand to the sky,
6
and swore by him who lives forever and ever, who created heaven and the things that are in it, the earth and the things that are in it, and the sea and the things that are in it, that there will no longer be delay,
7
but in the days of the voice of the seventh angel, when he is about to sound, then the mystery of God is finished, as he declared to his servants, the prophets.
8
The voice which I heard from heaven, again speaking with me, said, "Go, take the book which is open in the hand of the angel who stands on the sea and on the land."
9
I went to the angel, telling him to give me the little book. He said to me, "Take it, and eat it up. It will make your stomach bitter, but in your mouth it will be as sweet as honey."
10
I took the little book out of the angel's hand, and ate it up. It was as sweet as honey in my mouth. When I had eaten it, my stomach was made bitter.
11
They told me, "You must prophesy again over many peoples, nations, languages, and kings."
