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.

1295. That 'one man said to the next' means that this was started, that is, men started it, follows from the train of thought. This verse is dealing with the third state of the Church when falsities began to reign, in particular the falsities that arise from evil desires. There are two sources of falsities, the first being ignorance of the truth, the second evil desires. Falsity that arises from ignorance of the truth is not so harmful as falsity arising from evil desires, for the falsity of ignorance is the outcome either of having been so taught since early childhood, or of having been so preoccupied with various pursuits that one has never looked into whether something is true, or of not having been competent enough to judge what is true and what is false. Falsities that are the product of such ignorance do little harm provided that the person has not confirmed himself much in them, and so has not, on the instigation of some evil desire or other, persuaded himself to the point of defending those falsities. If he has he so intensifies the cloud of ignorance and converts it into darkness that he is incapable of seeing the truth.

[2] But falsity from evil desires comes into being when the origin of falsity is evil desire, that is, self-love and love of the world, as when somebody takes some point of doctrine, preaches it to captivate and take control of people's minds, and explains or twists that point of doctrine to his own advantage, and confirms it both by reasonings based on facts, and from the literal sense of the Word. The worship that results from this is unholy, however holy it may appear outwardly. For inwardly it is not worship of the Lord but worship of self. Nor does such a person acknowledge any truth, except insofar as he can explain it to his own advantage. Such worship is what is meant by 'Babel'. But the situation is different with people who, though born and brought up in such worship, do not know that it is falsity, and lead charitable lives. Their ignorance has innocence within it, and their worship has the good flowing from charity within it. The unholiness of worship is attributable not so much to the actual worship as to the nature of the worshipper.