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.

9874. 'Enclosed in gold shall they be in their settings' means that all of them in general and each in particular must emanate from the good belonging to the love that is received from the Lord and shown to the Lord. This is clear from the meaning of 'gold' as the good of love, dealt with in 113, 1551, 1552, 5658, 6914, 6917, 8932, 9490, 9510; and from the meaning of 'being enclosed in it in their settings' as emanating from that good. For every single stone was surrounded by and so enclosed in gold, and since 'gold' means the good of love what is enclosed means that which exists or emanates from that good. The like is meant in verse 11 of the present chapter by the sockets of gold which surrounded the two shoham stones placed on the shoulder-pieces of the ephod.

[2] The implications of all this are that the breastplate and its twelve stones represented every good and truth in the heavens and so represented all heaven, as shown above. Moreover not only the heavens but also every community in the heavens, indeed each angel within a community, is surrounded by a Divine sphere, which consists of Divine Good and Truth emanating from the Lord, see where this is dealt with in 9490-9492, 9498, 9499, 9534. And since the good and truth of this sphere is received by the angels, so also every single thing present with them emanates from there; for each angel is heaven in the smallest form it takes. The actual good emanating from the Lord is what the gold around the stones and enclosing them represents.

[3] The truth that this good is the good of love that is received from the Lord and shown to the Lord may be recognized from the consideration that all good belongs to love, for what a person loves he calls good and also feels to be such. From this it is evident that heavenly good is the good of love to the Lord, for this love is what joins angel and man to the Lord; through this love they are brought to Him and enjoy all the good of heaven. It is well known in the Church that this good comes from the Lord, for the teaching of the Church is that all good originates in God and none at all in oneself. From this it is evident that the good of love shown to the Lord must come from the Lord, and that good from any other source is not good.