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.

1047. That 'and when I cloud over the earth with cloud' means a time when, the will part of man's proprium being what it is, faith that flows from charity does not appear, is clear from what has been stated just above regarding 'the earth', that is, about the will part of man's proprium being such that it is constantly introducing obscurity, or falsity, into the understanding part. This is 'a clouding over'. All falsity originates in this way, as becomes quite clear from the fact that self-love and love of the world which belong to man's will are nothing else than hatred. For insofar as anyone loves himself he hates the neighbour. Because these loves are so contrary to heavenly love, it is inevitable that such things as are contrary to mutual love should be constantly flowing in from them. All of those things in the understanding part are falsities, and from them arises all the obscurity and darkness that is there. Falsity 'clouds over truth just as a dark cloud does to the light of the sun. And because falsity and truth cannot be present simultaneously any more than darkness and light, it follows plainly that one goes out as the other comes in. And this being an alternating occurrence reference is therefore made here to 'clouding over the earth with cloud', that is, a time when through the will part of the proprium no faith accompanying charity, or truth and derivative good, appears, still less good and derivative truth.