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.

8506. 'Six days you shall gather it' means the acceptance of truth before it is joined to good. This is clear from the meaning of 'six days' as states of conflict and labour, dealt with above in 8494, at this point states when truth is gaining acceptance, or states when good is being acquired through truth, dealt with immediately above in 8505; for during this state there is labour and conflict. During it a person is pitched into temptations, which are conflicts with the evils and falsities residing with him; at this time the Lord fights for him and also alongside him. But this state is followed by a state in which good and truth are joined together, a state that is accordingly one of rest also for the Lord. This state is what rest on the seventh or sabbath day represented. The reason why it is rest for the Lord is that when good has been joined to truth, a person abides in the Lord and is led by the Lord without labour or conflict. This state is what is meant by the second state spoken of immediately above in 8505.