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.

3845. 'Complete this week' means the continuance of the diligent effort. This is clear from the meaning of 'completing' here as serving or completing by serving, and so as meaning diligent effort, dealt with in 3824; and from the meaning of 'a week' as a state and also a whole period, dealt with in 728, 2044, in this case therefore the state and the subsequent period, and so a continuance. What has been said in 3814 about the meaning of 'a month' applies equally to the meaning of 'a week'. When used in the singular 'week' means the end of some previous state and the beginning of the one that follows it, and so a new state; and by the completing of this is meant from its beginning to its end. The reason why 'a week', like every other time-measurement specifically, means a state and also a period of time is that all states also have their own individual periods of the beginning, the continuance, and the end. In the next life however these are not perceived as periods of time but as states and their integral cycles. Here it is quite evident what 'a week' meant to the ancients, namely - in the proper sense - every period that was divided into seven phases, whether it was a period of seven days or of seven years or of seven ages, and so whether it was a long period or a short one. Here it is plainly a period of seven years. And because 'seven' with those people meant that which was holy, see 84-87, 395, 433, 716, 881, 'a week' therefore meant a holy period, and also the holiness of a period.