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.

575. 'The days of man were to be a hundred and twenty years' means that he ought to have remnants of faith. At verses 3, 4 of the previous chapter it was stated that 'days' and 'years' meant periods of time and states, and that the most ancient people meant states and changes in the states of the Church by the numbers which they compounded variously. The exact nature of their computation of things that had to do with the Church is one of those matters that have been lost. Here in like manner numbers of years occur, whose meaning nobody can possibly know unless he knows what is concealed in each of the numbers 1-12, and so on. It is quite apparent that they embody some arcanum or other, for in saying that they would live a hundred and twenty years this verse contradicts those that go before it. Nor subsequently did they live a mere hundred and twenty years, as is clear from what Chapter 11 says about those who lived after the Flood - that Shem lived 500 years after he beget Arpachshad, Arpachshad 407 years after he beget Shelah, Shelah 403 years as well after he beget Eber, and Eber 430 years after he beget Peleg. Chapter 9:28 says that Noah lived 350 years after the Flood; and other examples could be given. What the number 120 embodies however is clear merely from the numbers to and 12, for 120 is the product of 10 times 12. It means remnants of faith. In the Word the number ten has the same meaning as tenths, representing remnants which are preserved by the Lord within the internal man. And since these are the Lord's alone they are holy. The number twelve means faith, that is, all things belonging to faith in their entirety. So this composite number means remnants of faith.