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.

3114. 'There is both straw' means factual truths, 'and also much fodder with us' means the goods that go with these. This is clear from the meaning of 'straw' and 'fodder'. The reason 'straw' means factual truths is that it has reference to camels, whose food is such; for when 'camels' means the natural man as regards the general facts there, the food of the natural man - 'straw' - cannot have any other meaning, since no other food exists to sustain his life. The nourishment of the natural man is received from this, for if deprived of that food, that is to say, of knowledge, the natural man would cease to exist. The truth of this is evident from the life after death, for in that life spirits receive such things in place of food, see 56-58, 680, 681, 1480, 1695, 1973, 1974.

[2] Within the natural man, as within the rational man, there exist in general two kinds of things which constitute its essence - those that make up the understanding and those that make up the will. Truths belong to the things constituting the understanding, goods belong to those constituting the will. The truths present in the natural man are factual truths - that is to say, all the things housed in his external memory - and it is these that are meant by 'straw' when camels, and also horses, mules and asses are the subject. But the goods present in the natural man are chiefly the delights that go with an affection for those truths, and it is these goods that are meant by 'fodder'.