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.

8510. 'That some of the people went out to gather, and did not find' means that they wished to acquire it, but none was given. This is clear from the explanation above in 8505. The explanation there shows what actions springing from the truth of faith are and what those springing from the good of charity are. That is to say, it shows that a person whose actions spring from the truth of faith does not as yet have the order of heaven present in himself, whereas one whose actions spring from the good of charity does have it. For, according to order, when the Lord leads a person He does so through his will, consequently through good since this belongs properly to the will. In that case his understanding is subservient, and so is truth since this belongs properly to the understanding. When this state exists it is the sabbath, for then there is rest for the Lord; and the state comes into being once good has been joined to truth. That this state is the Lord's rest may be recognized from the statement that after six days of creation and labour Jehovah or the Lord rested on the seventh day from all His work, Genesis 2:2. This state is heaven's state, which explains why heaven itself is called the sabbath, and that in heaven the sabbath is perpetual. For the creation story in Genesis 1 serves to mean the new creation, which is regeneration, and the six days in the story serves to mean temptations and conflicts, see the explanations at Genesis 1, 2.