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.
Ezekiel 41:1-5
1
He brought me to the temple, and measured the posts, six cubits broad on the one side, and six cubits broad on the other side, which was the breadth of the tent.
2
The breadth of the entrance was ten cubits; and the sides of the entrance were five cubits on the one side, and five cubits on the other side: and he measured its length, forty cubits, and the breadth, twenty cubits.
3
Then went he inward, and measured each post of the entrance, two cubits; and the entrance, six cubits; and the breadth of the entrance, seven cubits.
4
He measured its length, twenty cubits, and the breadth, twenty cubits, before the temple: and he said to me, This is the most holy place.
5
Then he measured the wall of the house, six cubits; and the breadth of every side room, four cubits, all around the house on every side.
