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.

634. It is extremely difficult however to state intelligibly what the understanding of truth and the will for good are, properly speaking. The reason is that everything man thinks he ascribes to the understanding because he calls it so, and everything he desires he ascribes to the will because he calls it so. And it is even more difficult to state what they are in an intelligible way because the majority nowadays are also unaware that what belongs to the understanding is distinct and separate from what belongs to the will; for when they think something they say that they will it, and when they will something they say that they think it. Their speaking in this way is thus one reason for the difficulty. And a further reason why it is difficult to grasp the matter is that such people are engrossed solely in bodily interests, that is, their life consists in things of a more external nature.

[2] For these same reasons people are also unaware of the fact that with everybody there exists something interior, something more interior still, and indeed something inmost, and that the bodily and sensory part of a person is the most external. Desires and things of the memory are interior, affections and rational concepts more interior still, while the will for good and the understanding of truth are the inmost. Nothing could possibly be more distinct and separate than these are from one another, yet a bodily-minded man sees no difference at all, and so confuses them all with one another. This is the reason why he believes that when his physical body dies, everything else will die as well, when in reality only at that point does he start to live, and to do so indeed through his own interior things which are arranged in consecutive order. Unless man's interior things were distinct and separate in this way and arranged consecutively, men could not possibly be spirits, angelic spirits, or angels in the next life, all of whom differ in this way from one another according to things that are interior. Consequently the three heavens are very distinct and separate from one another. From all these considerations it now becomes clear to some extent what the understanding of truth and the will for good are, properly speaking, and that they are attributable only to the celestial man, or angels of the third heaven.