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.

84. Verses 2-3 And on the seventh day God finished His work which He had made, and rested on the seventh day from all His work which He had made. And God blessed the seventh day and made it holy, for on that day He rested from all His work which God had created when making it.

The celestial man is 'the seventh day'; and because throughout the six days it has been the Lord at work, that man is called 'His work'. In addition, because conflict at that point ceases, the Lord is said 'to rest from all His work'. It was for this reason that the seventh day was made holy and was called the Sabbath from a word for 'rest'; and it was in this manner that man was created, formed, and made, as may be seen plainly from the words themselves.