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.

521. 'He was no more for God took him' means that that doctrine reserved for use by descendants. He was no more. As regards Enoch, that which the Most Ancient Church had perceived, as has been stated, was converted by him into doctrine, something that had not been allowed to people of that period. For knowing something from perception is altogether different from learning it from doctrine. People who have perception have no need to learn through the channel of formulated doctrine what they know already. Take, for the sake of illustration, someone who knows already how to think clearly. He has no need to learn rules on how to think. If he did so his ability to think clearly would perish, as happens to people buried in the dust of sheer intellectualism. In the case of people whose knowledge comes from perception, the Lord grants them to know what good and truth are through an internal channel, while those who learn from doctrine are granted it by an external channel, that is, by way of the physical senses. The difference between the two is like that between light and darkness. Furthermore the perceptions of the celestial man lie beyond all description, for they enter into the smallest details and are for ever varied according to states and attendant circumstances. Now as it was foreseen that the perceptivity of the Most Ancient Church would perish, and that subsequently people would learn what truth and good were by means of doctrines, that is, they would come to the light by way of darkness, it is therefore said here that 'God took him', which is to say, He preserved such doctrine for the use of descendants.