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.

9668. 'And you shall set up the dwelling-place according to the plan which you saw on the mountain' means in the four quarters, according to the states of good and of the truth springing from it in the heaven that is represented. This is clear from the meaning of 'the dwelling-place' as a representative of heaven, dealt with in 9594; and from the meaning of 'according to the plan which you saw on the mountain' as to the four quarters according to the states of good and of the truth springing from it in heaven. This is what 'the plan according to which the dwelling-place was to be set up' serves to mean, Mount Sinai where it was seen being heaven, see 9420. From the description that is given it is evident that the dwelling-place was positioned lengthways from east to west, and that the entrance was at the east end and the ark at the west. The sides therefore were to the south and to the north. The eastern quarter of the dwelling-place represented the state of good on the rise, the western quarter the state of good on the decline, the southern quarter the state of truth dwelling in light, and the northern quarter the state of truth dwelling in shade.

[2] The entrance was towards the eastern quarter because the Lord enters heaven by way of the good of love, as also becomes clear in Ezekiel, where the new temple is the subject and the following words occur,

He brought me to the gate which was facing towards the east, and behold, the glory of the God of Israel came from the way of the east. And the glory of Jehovah entered the house by the way of the gate, the face of which is towards the east; and the glory of Jehovah filled the house. Ezekiel 43:1-6.

And after this,

Jehovah said to me, The gate facing east shall be shut and not opened, and no man (vir) shall enter by it; but Jehovah, the God of Israel, will enter by it. Ezekiel 44:1-2.

From these words it is plainly evident that the Lord alone enters heaven by way of the good of love, and that the good of love from the Lord fills heaven and composes it. 'The east' means the Lord in respect of the good of love because the Lord is the Sun of heaven, 3636, 3643, 7078, 7083, 7171, 8644, 8812. But in heaven things are positioned in such a way that the east is where the Lord appears as the Sun, straight before the right eye, 4321 (end), 7078, 7171. From there towards the west, thus in a direct line from the east and towards the west, are those with whom the good of love is present; but to the south are those dwelling in the light of truth, and to the north those in the shade of truth. All who are in heaven look towards the Lord, for to look frontwards there is to look towards Him. No one there can look backwards away from Him, no matter how far they turn themselves about, see 4321 (end). But this is an arcanum which the natural man cannot take in. These are the kinds of things that were represented by the plan seen by Moses on the mountain, according to which the dwelling-place was to be set up.