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.

1049. 'And I will remember My covenant which is between Me and you' means the Lord's mercy specifically towards those who have been regenerated, and those who are able to be. This too follows from what has been said above, for with the Lord 'remembering' is the same as showing mercy. The act of remembering cannot be attributed to the Lord since He knows from eternity every single thing. But having mercy can be attributed since He knows man's nature, namely that his proprium, as stated, is hell-like and constitutes his particular hell. For through the will side of his proprium he communicates with hell; and from hell and of itself that proprium is such that man has no greater or stronger desire than to plunge headlong into hell. Nor is he satisfied with that but desires [to take] everyone in the whole world [with him]. Because man of himself is by nature such a devil, and the Lord knows it, 'remembering the covenant' consequently means nothing else than having mercy, regenerating through Divine means, and by a mighty power drawing man towards heaven, insofar as he is such as to make this possible.