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.

196. In ancient times people who relied on sensory evidence rather than matters of revelation were called serpents. Nowadays the position is even worse, for not only are there people who believe nothing unless they can see it with their eyes and apprehend it with their senses, there are also those who confirm themselves in that attitude by means of facts unknown to the most ancient people, and who in so doing blind themselves very much more. To make known how people who draw conclusions about heavenly things on the basis of sensory evidence, facts, and philosophical arguments, so blind themselves that they subsequently see and hear absolutely nothing, and who are not only the deaf serpents but also the far more deadly flying serpents, mentioned in the Word as well, let their belief concerning the spirit serve as an example.

[2] Anybody who is sensory-minded, that is, whose belief is rooted solely in the senses, denies the existence of the spirit because he does not see it. He says, 'Because I do not feel it, it is nothing; what I see and touch, I know to exist'. Anybody who is factually-minded, that is, who bases his conclusions on factual knowledge, says, 'What is the spirit but perhaps breath, or vital heat, or something else known to me, which is dissipated when it comes to an end? Do not animals as well have a body, and senses, and something analogous to reason? Yet people say that animals are destined to die but man's spirit to live.' In this way they deny the existence of the spirit. Philosophers, men wishing to be more incisive than everybody else, speak of the spirit in terms which they themselves are not clear about since they argue about them. They contend that not a single expression is applicable which in any way derives from what is material, organic, or spatial. In this way they dismiss the spirit from their ideas, and as a result it passes from their notice and becomes nothing at all.

[3] Those among them however who are more sensible say that the spirit is thought, but when they begin to reason about thought they at length conclude, since they separate thought from substance, that it will disappear when the body breathes its last. In this way everyone who reasons on the basis of sensory evidence, facts, and philosophical arguments denies the existence of the spirit, and in denying its existence never believes anything that is said about the spirit or about spiritual things. But if indeed the simple in heart are questioned they say that they know that the spirit exists because the Lord has said that they will live after death. Instead of smothering their rationality they nurture it by means of the Word of the Lord.