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.

628. And the angel stood near, saying, signifies the Lord's will and command. This is evident from the signification of "to stand near," as being here will (of which presently); also from the signification of "angel," as being the Lord in respect to the Word (See above, n. 593); also from the signification of "saying," as being, when the Lord speaks, command; for what the Lord says is to be done, or that anyone should do it, is a command. "The angel stood near" means here the Lord's will, because in the spiritual world the thought, with the purpose, and will to see another, to speak with him, and to give him a command, causes the other to be present, that is, to stand near; for in that world there are no distances that are constant and thence measurable, as in the natural world; but likeness of affection and of thought thence causes presence, and unlikeness of these causes absence; this is the origin of all distances in that world. This comes from the universal principle that the Lord is present with all according to their love to Him and according to their love towards the neighbor and thoughts from these. From this universal principle all distances, that is, all presence and absence among angels and spirits, exist; therefore when anyone desires to speak with another, that is, thinks about him from a purpose or wish to speak with him, that one immediately becomes present, or he is present with that one. (That this is so can be seen in the work on Heaven and Hell, n. 191-199, where Space in Heaven is treated of.) From this it can now be seen why the Lord's will is signified by "the angel stood near," for "to stand near" means to be present.