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.

649. But here the numbers, or the measurements of the ark, mean nothing other than the remnants that existed with the member of this Church when he was being reformed, and in particular that they were few. This is clear from the fact that among those numbers five is predominant, which in the Word means some, or a little, as in Isaiah,

Gleanings will be left in it, as the shaking of an olive tree, two or three berries on the top of the highest branch, four or five on the branches of a fruitful tree. Isaiah 17:6.

Here 'two or three' and 'five' stands for a few. In the same prophet,

One thousand at the rebuke of one, at the rebuke of five will you flee, until you are left like a flagstaff on top of a mountain. Isaiah 30:17.

This also stands for a few. The minimum penalty when making restitution was a fifth part, Leviticus 5:16; 6:5; 22:14; Numbers 5:7, and the minimum increase when they redeemed an animal, a house, a field, or tithes, was a fifth part, Leviticus 27:13, 15, 19, 31.