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.

9673. 'With cherubs' means watchfulness, guarding against the intermingling of the two - of spiritual good and celestial good, and so of the middle heaven and the inmost heaven. This is clear from the meaning of 'cherubs' as watchfulness and providence, guarding against access to the Lord except through good and guarding against any harm being done to the good from the Lord which is present in heaven and with a person, dealt with in 9509. The reason why guarding against the intermingling of spiritual good and celestial good, and so of those two heavens, is also meant is that if they did become intermingled they would both suffer harm, harm so serious that those actual heavens would be destroyed, as may be recognized from the difference between the two kinds of good, and so between the two heavens, spoken about in the places referred to above in 9670. For this reason there are intermediate angelic communities, among whom celestial-spiritual good exists and spiritual-celestial good; and through these communities the two heavens are linked together, 9670, 9671. Even with these communities the two forms of good have not been joined together, but exist distinct and separate from each other. From all this it is evident that those communities are guards, protecting both kinds of good from becoming intermingled, and therefore also that this watchfulness and providence which are the Lord's are meant by 'cherubs'.