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.

2850. 'And like the sand which is on the seashore' means a whole multitude of correspondent facts. This is clear from the meaning of 'the sea' as facts in general or a gathering together of them, dealt with in 28, 2120, and from the meaning of 'the sand' as facts individually and separately. Facts are compared to 'the sand' because in the internal sense the particles of stone from which the sand is formed mean facts, 643, 1298. Both comparisons are made here - that they will be multiplied 'as the stars of the heavens' and 'as the sand on the seashore 'because 'stars', or cognitions, are related to the rational, whereas 'the sand of the seashore', or facts, are related to the natural. When the things that belong to the rational man, namely the goods and truths present in cognitions, so exist in accordance with the things that belong to the natural man, that is to say, with facts, that they make one or mutually support each other, they in that case correspond. The Lord brings man's rational concepts and his natural images into this state of correspondence when He regenerates him, that is, makes him spiritual. It is for this reason that both the stars of the heavens and the sand on the seashore are mentioned here. Otherwise one phrase would have been sufficient.