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.

10106. 'And Aaron and his sons shall eat the flesh of the ram' means the making of spiritual good their own by those who are at home with internal things, from the Lord. This is clear from the meaning of 'eating' as making one's own, dealt with in 3168, 3513(end), 3596, 4745; from the representation of 'Aaron and his sons' as the Lord in respect of Divine Good and Divine Truth, dealt with in 9806, 9807, 10068, at this point that Good and Truth in the heavens since 'filling the hand', which means the transmission and the reception of them, is the subject; and from the meaning of 'the flesh of the ram' as the good of the internal man or good on internal levels ('flesh' means good, see 3813, 7850, 9127, and 'the ram' the good of innocence and charity in the internal man, 9991, 10042). The words 'the Lord in respect of Divine Good and Divine Truth in the heavens, and the transmission and the reception of that Good and Truth there' are used because the Lord is above the heavens. He is the Sun of heaven and is also actually seen by those in heaven as the Sun; and all the light the heavens have comes from this. By means of the light and heat from it He is present in the heavens, so much so that He seems to be present there entirely; for He fills the heavens and constitutes them. The light radiating from Him as the Sun is essentially Divine Truth, from which angels derive the wisdom and intelligence they possess. And the heat radiating from Him as the Sun is the Divine Good of His Divine Love there. The transmission and the reception of this Divine Good and that Divine Truth in the heavens is what 'filling the hand' means.

The Lord is the Sun of heaven, the source of the light and heat in the heavens from which angels derive the life, that is, the wisdom and love, they possess, see 3636, 3643, 4321(end), 5097, 7078, 7083, 7171, 7173, 7270, 8644, 8812.