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.

1249. That these words mean the spread of the worship, even from the truths of faith to the good that stems from charity, 'Mesha' meaning truth, 'Sephar' good, cannot indeed be confirmed from the Word as no mention is made of Mesha or Sephar in the Prophets. Nevertheless the matter becomes clear from the fact that this verse stands as the conclusion to what has gone before, and especially from the fact that 'the mountain of the east' is the ultimate bound to which the preceding things look, and the fact that in the Word 'the mountain of the east' means charity from the Lord, as becomes clear from what follows below. The matter also becomes clear from the fact that all things of the Church look towards charity as their ultimate goal and end in view. From all this it follows that 'Mesha' means truth, or the boundary from which the thing begins, while 'Sephar' means good and so charity, meant by 'the mountain of the east', or the boundary to which it is directed.