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.

1179. 'Therefore it used to be said, Like Nimrod, mighty in hunting before Jehovah' means that since so many were being persuaded this became a proverbial expression. In addition to this it means that such a religion easily captivates people's minds. This becomes clear from what has been stated and also from the actual sense of the letter. Moreover because in early times names were given to real things this name was given to this form of worship, that is to say, it used to be said that 'Nimrod', meaning that form of worship, was 'mighty in hunting', that is, was one that captivated people's minds. The reason for the words 'before Jehovah' is that people with whom that form of worship existed called separated faith 'Jehovah' or 'a man Jehovah', as is clear from what has been stated already about Cain in 340, who likewise means faith separated from charity. The difference between Cain and Ham however lies in the fact that the former existed in the celestial Church which had perception, while the latter existed in the spiritual Church which had none. As a consequence the former was far more monstrous than the latter. In early times such people were called 'mighty', as in Isaiah,

All the glory of Kedar will be brought to an end, and the remainder of the number of the bows of the mighty men of the sons of Kedar will be diminished. Isaiah 21:16-17.

And in Hosea,

You have ploughed wickedness, you have reaped iniquity, you have eaten the fruit of lying, because you have trusted in your way, in the multitude of your mighty men. Hosea 10:13.

And in other places. They called themselves 'men' (vir) and 'mighty' from faith, for one particular word in the original language expresses the idea of being 'mighty' and at the same time that of a man (vir), and it is used in the Word in reference to faith, and indeed in both senses.