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.

6238. 'As Reuben and Simeon will they be mine' means that they will be truth and the good of truth. This is clear from the representation of 'Reuben' as faith in the understanding, and the truth of doctrine by means of which one is able to arrive at the good of life, dealt with in 3861, 3866, thus in general truth that belongs to the understanding; and from the representation of 'Simeon' as faith in the will, consequently truth realized in action, which is the good of faith or the good of truth, dealt with in 3869-3872, 4497, 4502, 4503, 5626, 5630, thus in general good which belongs to the new will. The things that Ephraim and Manasseh represent are clearly similar to all this.

[2] But since Reuben profaned what he represented, 4601, and Simeon polluted what he represented, 4497, 4501, 4507, on account of which they were cursed, see verses 3-7 of the next chapter, they lost their birthright and Joseph's sons Ephraim and Manasseh were acknowledged as the firstborn in place of them, 1 Chronicles 5:1. Nevertheless what Reuben and Simeon represented remained with them, for it makes no difference what the character of the person who serves to represent something is like, 665, l097 (end), 4281. That is to say, the representation of faith in the understanding remained with Reuben, and the representation of faith in the will with Simeon. But what resided with Ephraim was the representation of the Church's understanding, and what resided with Manasseh was the representation of the Church's will.