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.

561. But what are remnants? Not only the goods and truths which one has learned from the Word of the Lord from early childhood onwards and so had imprinted in his memory, but also all resulting states, such as states of innocence from early childhood; states of love towards parents, brothers and sisters, teachers, and friends; states of charity towards the neighbour, and also of compassion on the poor and needy; in short, all states involving good and truth. These states, together with the goods and truths that have been imprinted in the memory, are called remnants, which the Lord preserves with a person and stores away in his internal man, though the person himself is not at all directly conscious of this. Here they are separated completely from the things that are the person's own, that is, evils and falsities. The Lord so preserves all of those states with the individual that not even the least of them perishes. I have been given to know this from the fact that every one of man's states from his infancy right through to extreme old age not only carries over into the next life but also reappears. Indeed those states are exactly the same as when he lived in the world. Thus not only are goods and truths in the memory carried over, but also all states of innocence and charity. And when states of evil and falsity, or wickedness and false notions, recur - for every single one of these as well, down to the smallest detail, is carried over and reappears - they are at that time moderated by the Lord through those states of innocence and charity. From all this it becomes clear that if a person possessed no remnants, he would inevitably be subject to eternal condemnation. See what has appeared already in 468.