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.

3494. 'And he called Esau his elder son' means the affection for natural good, or the good of life. This is clear from the representation of 'Esau' as the Divine Good of the Natural, dealt with in 3300, 3302, 3322. And because the good of the natural is that which manifests itself in affection and life, it is accordingly the affection for natural good, or the good of life, that 'Esau' represents here. The affection for good in the natural and consequently the good of life is that which is called 'the elder son', whereas the affection for truth and consequently the doctrine of truth is that which is called 'the younger son'. The fact that the affection for good and consequently the good of life is the elder son, that is, the firstborn, is quite evident from the consideration that good reigns in anyone's children at first. Indeed they are in a state of innocence, and a state of love towards their parents or nursemaid, and a state of mutual charity towards playmates, so that good is the firstborn with everyone. This good which is fostered in this state within a person when he is a small child remains with him, for whatever is instilled in infancy acquires life to itself; and because it remains it becomes the good of life. Indeed if a person were devoid of such good as he has had with him from earliest childhood he would not be human but would be more savage than any wild animal of the forest. Not that its presence is apparent, for everything that has been instilled in earliest childhood inevitably appears to be something natural, as is quite evident from being able to walk, from all our other bodily movements, and from the right and proper ways to behave among other people; also from being able to talk, and from so many other abilities. From this it may be seen that good is 'the elder son', that is, the firstborn, and truth therefore 'the younger son', or one born later, for truth is not learned until childhood, adolescent, and adult years are reached.

[2] Each of them, good and truth in the natural or external man, is 'a son', that is to say, a son of the rational or internal man, for whatever comes into being in the natural or external man flows in from the rational or internal man, and from there comes into being and is born. That which does not come into being and is not born from there is not living and human, but is like what you might call body and senses without a soul. Hence both good and truth are called 'sons', and indeed sons of the rational. Yet it is not the rational that produces and gives birth to the natural, but an influx by way of the rational into the natural, an influx coming from the Lord. His 'sons' therefore are all the young children who are born, and after that time whenever they become wise. Also, insofar as the latter are at that time 'young children' - that is, insofar as the innocence of a young child, the love of a child for its parent (who is now the Lord), and mutual charity towards playmates (who are now the neighbour) exist in them, they are adopted by the Lord as 'sons'.