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.

8935. 'An altar of soil you shall make for Me' means something which in a general way represents worship that springs from good. This is clear from the meaning of 'an altar' as the chief representative of the Lord and consequently of the worship of Him, dealt with in 921, 2777, 2811, 4489, 4541; and from the meaning of 'soil' or 'the ground' as good. The reason why 'the ground' stands for good is that a Church which abides in good is meant by 'the ground', 566. This is why Adam was said to be formed from the ground, at Genesis 2:7; 3:19; for one who belonged to the celestial Church, that is, the Church itself abiding in good, was meant by him, 478, 479. Since there are two entities that inspire worship of the Lord - goodness and truth - worship springing from good was represented by an altar of soil, while worship springing from truth was represented by an altar of stones. Both kinds of altars are the subject here; and those two entities that inspire worship are called faith and charity. Worship springing from truth has connection with faith, and worship springing from good has connection with charity.

[2] As regards worship inspired by faith and worship inspired by charity, or that which springs from truth and that which springs from good, the situation is that before a person has been regenerated his worship consists of truth, but after he has been regenerated it consists of good. Before a person has been regenerated he is led by means of truth to good, that is, by means of faith to charity; but when he has been regenerated he abides in good and consequently in truth, that is, in charity and consequently in faith, 8516, 8539, 8643, 8648, 8658. These two kinds of worship are what the altars of soil and of stone represent. The reason why the altar is the chief representative of worship of the Lord is that burnt offerings and sacrifices were offered on it; and it was in these that the worship of God by the Hebrew nation, and subsequently by the Israelite and Jewish nation, chiefly consisted, 923, 1343, 2180, 2805, 2807, 2830, 3519, 6405.