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.

8991. 'And he shall serve him forever' means into eternity. This is clear from the meaning of 'serving' as obeying, for slaves are those who obey and masters those who give orders (the fact that those represented by 'slaves' are those who did what was good from obedience, not from charitable affection, is clear from things stated previously); and from the meaning of 'forever' as into eternity. In the sense of the letter 'forever' here means servitude with his master to the end of his life; but in the internal or spiritual sense it means into eternity, because it refers to the state after death. The expression 'into eternity' is used because those who do what is good from faithful obedience and not from charitable affection, who were represented by 'slaves', cannot ever be brought in the next life to a state of good, that is, so that their actions spring from good; for everyone's life remains with them after death. What a person is like when he dies, so he continues to be, in keeping with the common saying, Where the tree falls, there it lies. Not that he continues to be what he is like around the time of death but what the whole course of his life has made him like when he dies. Therefore those who have learned during life in the world to do what is good solely from obedience, not from charity, remain the same into eternity. They are perfected in obedience, it is true, but they do not attain any degree of charity.