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.

171. There is no way to describe briefly how things look to angels in the heavens. To a considerable extent, they look like the things we see on earth, but they are more perfect in form and also more abundant.

We may conclude that there are things like this in the heavens because of what the prophets saw - for example what Ezekiel saw of the new temple and the new earth as described in chapters 40-48 [of his book], what Daniel describes in his chapters 7-12, what John saw as described from the first through the last chapter of Revelation, along with other visions presented in both the historical and the prophetic books of the Word. They saw things like this when heaven was opened to them, and heaven is said to be opened when our inner sight, the sight of our spirit, is opened. For the things that exist in heaven cannot be seen with our physical eyes, but only with the eyes of our spirit; and when it pleases the Lord, these are opened. At such times we are led out of the natural light that our physical senses are in and raised into the spiritual light in which we dwell because of our spirit. This is the light in which I have seen the things that exist in the heavens.