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.

6314. As it is with light, so it is with the heat that is vital to a person's existence. In no way at all does that vital heat originate in the heat radiated by the sun in the world; rather, it originates in spiritual heat, which is love and radiates from the Lord. This heat is what angels have. Consequently the amount of vital heat a person has is determined by the amount of love he has. His body however is warmed by worldly heat, as is his sensory awareness within it; yet vital heat flows into that worldly heat and imparts life to it. Differences in the purity and intensity of this heat are akin to variations in the clarity of the light. This vital heat is what one should understand by the holy fires mentioned in the Word, and therefore heavenly loves are meant by those fires. But in a contrary sense that worldly heat is what one should understand by the fires of hell, and therefore hellish loves and the desires that go with them are meant when these fires are mentioned in the Word.