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.

998. Verse 13. And I saw out of the mouth of the dragon and out of the mouth of the beast, signifies from the thought, reasoning, religion, and doctrine of those who are in faith alone, and in confirmations of that faith from the natural man. This is evident from the signification of the "mouth," as being thought, reasoning, religion, and doctrine (See n. 580, 782, 794); also from the signification of the "dragon," as being those who are in faith alone both as to doctrine and as to life (See n. 714-715, 716, 737); also from the signification of the "beast," as being those who confirm faith alone by reasonings from the natural man (See n. 773). For there were two beasts, one coming up out of the sea, the other out of the earth; and "the beast out of the sea" means faith alone confirmed by reasonings from the natural man; and "the beast out of the earth" means that faith confirmed from the sense of the letter of the Word, and its consequent falsification. But here "the beast coming up out of the sea" is meant, that is, faith confirmed by reasonings; since it is added, "out of the mouth of the false prophet," and "the false prophet" has the same signification as "the beast out of the earth," namely, faith alone confirmed by the Word, thus the doctrine of falsity from falsified truths.

[2] This and what here follows describes how the doctrine of faith alone has blunted and almost extinguished the faculty of understanding the Divine truth, which is given to every man by the Lord so far as falsities from evil do not block up influx and access, lest anything should be perceived from heaven. For a man is like a garden, which receives light in winter equally as in summer, but not heat; and yet when it does receive heat it blossoms and bears fruit. So the evil man equally with the good man is able to receive light, that is, to understand Divine truth, but he cannot blossom and become fruitful, that is, be wise and do works that are good, except as he receives heat, that is, the good of love.

[3] There are many who believe that the learned, since they know many things from the Word and from doctrine from the Word, are more intelligent and wise than others; and yet they have only so much intelligence and wisdom as they have spiritual heat, that is, the good of love, for only so far is their faculty of understanding truths opened and vivified; while that faculty is as it were covered up and blotted out by the evils of one's own love. That they have, nevertheless, this intellectual faculty, however covered up and blotted out, I have frequently heard proved by experiment. There were spirits who were wholly in falsities from evil, and who in their heart denied the Divine influx into all things of the understanding of truth and of the will of good, that is, denied the Divine Providence, and thus they confirmed with themselves that all things are of nature and of their own prudence. Although these spirits had as it were no faculty of understanding truths when they thought about them with themselves, yet when they heard from others that the Divine is everything, and that the natural is relatively nothing except as a tool is to a workman, they understood all this as clearly as those did who taught it, or as others did who had confirmed themselves in that Divine truth. But the moment they turned away their ear they fell back into things contrary, and no longer understood these truths, because they covered them up with falsities from confirmations. This clearly showed that all have the faculty of understanding the truth or of receiving light from heaven; and yet they receive only so far as by life they are in the good of love; the same as a garden that admits light from the sun in winter equally as in summer, and yet it blossoms and bears fruit only so far as it receives at the same time heat from the sun, which it does in spring and summer.

(Continuation respecting the Sixth Commandment)

[4] Man has such and so much of intelligence and wisdom as he has of conjugial love. The reason is that conjugial love descends from the love of good and truth as an effect does from its cause, or as the natural from its spiritual; and from the marriage of good and truth the angels of the three heavens have all their intelligence and wisdom; for intelligence and wisdom are nothing else than the reception of light and heat from the Lord as a sun, that is, the reception of Divine truth conjoined to Divine good, and of Divine good conjoined to Divine truth; thus it is the marriage of good and truth from the Lord. That it is so has been made clearly evident by angels in the heavens. When these are separated from their consorts they are indeed in intelligence, but not in wisdom; but when they are with their consorts they are also in wisdom; and what is wonderful, as they turn the face to their consort they are to the same extent in a state of wisdom; for the conjunction of truth and good is effected in the spiritual world by looking; and the wife there is good and the husband truth; therefore as truth turns itself to good so truth becomes living. By intelligence and wisdom ingenuity in reasoning about truths and goods is not meant, but the faculty of seeing and understanding truths and goods, and this faculty man has from the Lord.