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.

911. 21:18 The construction of its wall was of jasper. This symbolically means that with people of the New Church the Divine truth of the Word in its spiritual sense shines through every Divine truth in the literal sense.

The wall symbolizes the Word in its literal sense (no. 898). Its structure symbolizes everything in it, because everything in it is in its structure. Jasper has, on the whole, the same symbolic meaning as any precious stone, and in application to the Word a precious stone symbolizes the Divine truth of the Word in its literal sense through which the Divine truth in its spiritual sense shines through (nos. 231, 540, 726, 823). That jasper has the same symbolic meaning may be seen in no. 897 above.

The Divine truth shines through because Divine truth in the literal sense is seen in natural light, whereas Divine truth in the spiritual sense is seen in spiritual light. Consequently, when spiritual light flows into the natural sight in a person who is reading the Word, it enlightens him and he sees truths in it, for the objects of spiritual light are truths. The Word in its literal sense is also such that the more enlightened a person is by the influx of the light of heaven, the more truths he sees because of their connection and consequent form; and the more truths he sees in this way, the more interiorly is his rationality opened. For the rational faculty is the indispensable receptacle of heavenly light.