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.
Deuteronomy 33:8-11
8
Of Levi he said, "Your Thummim and your Urim are with your godly one, whom you proved at Massah, with whom you strove at the waters of Meribah;
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who said of his father, and of his mother, 'I have not seen him;' Neither did he acknowledge his brothers, Nor did he know his own children: For they have observed your word, and keep your covenant.
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They shall teach Jacob your ordinances, and Israel your law. They shall put incense before you, and whole burnt offering on your altar.
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Yahweh, bless his substance. Accept the work of his hands. Strike through the hips of those who rise up against him, of those who hate him, that they not rise again."
