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.

10001. 'You shall bring to the door of the tent of meeting' means the joining together of the two, in heaven. This is clear from the meaning of 'bringing' as presence and togetherness, as above in 9997, at this point the joining together of Divine Good and Divine Truth from the Lord, in heaven; from the meaning of 'the door' as introduction, dealt with in 8989; and from the representation of 'the tent of meeting' as heaven, dealt with in 9457, 9481, 9485, 9963. The reason why the young bull and the rams, the unleavened bread, cakes, and wafers in the basket, and Aaron together with his sons were presented at the door of the tent of meeting, and why the latter were then clothed with the garments and anointed there, and the former were offered on the altar there, was that the place where the door of the tent of meeting was, represented the marriage of Divine Good to Divine Truth. For the altar, which also was positioned before the door of the tent, represented the Lord in respect of Divine Good, while the tent of meeting represented the Lord in respect of Divine Truth. Consequently the place before the door of the tent represented the joining together of goodness and truth, a joining together that is called the heavenly marriage. Regarding the altar of burnt offering, that it represented the Lord in respect of Divine Good, see 9964; and regarding the tent of meeting, that it represented the Lord in respect of Divine Truth, 9963. The fact that the altar was positioned there is clear in Moses,

And Moses placed the altar of burnt offering at the door of the tent. Exodus 40:29.

As for the joining together of goodness and truth, that this is the heavenly marriage, and that in that marriage heaven consists, see 2173, 2508, 2618, 2803, 3004 and following paragraphs, 3132, 952, 4434, 6179. From all this it is now evident that bringing Aaron and his sons to the door of the tent of meeting means the joining together of the two, that is to say, of Divine Good and Divine Truth from the Lord, in heaven.