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.

2227. That 'Abraham will certainly become a great and numerous nation' means that from the Lord will come all good, and from this all truth, is clear from the representation of 'Abraham' as the Lord, dealt with often above, and also from the meaning of 'nation' as good, dealt with in 1159, 1258-1260, 1416, 1849 - here 'a great and numerous nation', by which good and truth deriving from it are meant. That 'great' has reference to good and 'numerous' to truth may become clear from other places in the Word, but I must refrain from quoting them here. Truth derived from good is in the genuine sense spiritual good, for there are two kinds of good, quite distinct and separate from each other, namely celestial good and spiritual good. Celestial good is the manifestation of love to the Lord, spiritual good the manifestation of love towards the neighbour. From the former, that is, celestial good, flows the latter, that is, spiritual good; for no one is able to love the Lord unless he also loves the neighbour. Love to the Lord embraces love towards the neighbour, for love to the Lord originates in the Lord, thus in Love itself towards the whole human race. Abiding in love to the Lord is the same as abiding in the Lord, and one who abides in the Lord must inevitably abide in His love which is directed towards the human race, and thus towards the neighbour, so that in him both kinds of good are present - both celestial and spiritual. Celestial good is indeed good itself, but spiritual good is in fact truth that goes with or is derived from that celestial good. This truth, as stated, is spiritual good. It is celestial good that is meant by 'great', but spiritual good by 'numerous'.