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.

909. 21:17 Then he measured its wall, a hundred and forty-four cubits. This symbolically means that John was shown the nature of the Word in that church, that from it come all of its truths and goods.

The angel's measuring means, symbolically, that its character was shown, as in no. 907 above. The wall symbolizes the Word in its literal sense (no. 898). A hundred and forty-four symbolizes all the truths and goods of the church from the Word (no. 348). Cubits symbolize character, like any unit of measure.

The number 144, in fact, has the same symbolic meaning as the number 12, because the product of 12 multiplied by 12, or 12 squared, is the number 144, and multiplication does not take away its symbolism.