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.

482. Until now nobody has known what the years and the numbers of years occurring in this chapter mean in the internal sense. People who stay within the sense of the letter imagine that they are no more than chronological years. But none of the content from here down to Chapter 12 is history as it seems to be in the sense of the letter, for every single detail contains something of a different nature. What applies to names applies to numbers as well. In the Word the number three occurs frequently, and so does the number seven; and in every instance they mean something holy or inviolable as regards those states which the periods of time or whatever else that is mentioned embody or represent. This applies as much to the shortest as to the longest time-intervals; for just as parts makeup the whole, so do the shortest make up the longest. For a similarity must exist in order that a whole may emerge satisfactorily out of the parts, or that which is largest out of that which is smallest.

[2] As in Isaiah,

Jehovah has now spoken, saying, In three years, according to the years of a hireling, the glory of Moab will be rendered worthless. Isaiah 16:14.

In the same prophet,

The Lord said to me, Within yet a year, according to the years of a hireling, and all the glory of Kedar will be brought to an end. Isaiah 21:6.

Here both the shortest as well as the longest time-intervals are meant. In Habakkuk,

O Jehovah, I have heard Your fame; I was afraid. O Jehovah, revive Your work in the midst of the years, in the midst of the years do You make it known. Habakkuk 3:2.

Here 'the midst of the years' stands for the Lord's Coming. If the intervals are shorter this stands for every coming of the Lord, as when a person is being regenerated; but if longer it stands for the rising anew of the Lord's Church. It is also called in Isaiah 'the year of the redeemed', The day of vengeance was in My heart, and the year of My redeemed has come. Isaiah 63:4.

So too 'the thousand years' for which Satan is to be bound, Revelation 20:2-3, 7, and 'the thousand years' associated with the first resurrection, Revelation 20:4-6. These in no way mean a thousand years but the states associated with them. For just as 'days, as shown 'already, are interpreted as a state, so too are 'years', and the states are described by the number of the years. From this it becomes clear that periods of time in this chapter also embody states, for every Church experienced a different state of perception from the next, according to differences of disposition resulting from inherited and acquired characteristics.