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.

6013. 'And their young children' means accompanied by aspects of innocence. This is clear from the meaning of 'young children' as innocence, dealt with in 3183, 5608. Natural truth is said to be accompanied by aspects of innocence, and also those of charity, for the reason that if it lacks innocence and charity spiritual truths cannot cause it to move ahead. For to be genuine, truth must derive its essence and life from charity, and charity must derive its essence and life from innocence. Indeed these inner virtues which give truth its life follow one another in this order: What is inmost is innocence; below this comes charity; and what is lowest is a charitable deed motivated by or done in conformity with truth. The reason why they follow one another in this way is that they follow one another in heaven in that order. The inmost or third heaven is the heaven of innocence; the middle or second heaven is the heaven of charity that has innocence from the inmost heaven within it; and the lowest or first heaven is the heaven of truth that has charity from the second heaven within it, and innocence from the third within that charity.

[2] These virtues ought to exist in the same order in man, for interiorly the human being is so created that he conforms to an image of the three heavens. This also is why a person who has been regenerated is an individual heaven or the smallest form heaven can take. But exteriorly, especially so far as his body is concerned, he is so created that he conforms to an image of the world, which was why the ancients called him the microcosm. The ear has been made to conform to the whole nature of air and sound; the eye to conform to the whole nature of the ether and light; the tongue to every perception of particles dissolved and suspended in fluids; the nostrils to the perception of particles suspended in the atmosphere; touch to the perception of cold and heat, and also of weights; and so on. Just as a person's external senses conform to an entire image of the natural world, so his internal senses, which are those of his understanding and will, have been so created that they conform to an entire image of heaven. They have been created like this so that the human being as an individual, like heaven as a general whole, may be a recipient of Divine Good from the Lord.