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.

53. Of the fallacies of the senses, in which merely natural and sensual men are, mentioned above in this doctrine (n. 45). Merely natural and sensual men think and reason from the fallacies of the senses (n. 5084, 5700, 6948-6949, 7693). Of what quality the fallacies of the senses are (n. 5084, 5094, 6400, 6948). To which the following particulars shall be added. There are fallacies of the senses in things natural, civil, moral, and spiritual, and many in each of them; but here I design to recite some of the fallacies in spiritual things. He who thinks from the fallacies of the senses, cannot understand:

(1.) That man after death can appear as a man; nor that he can enjoy his senses as before; nor consequently that angels have such a capacity.

(2.) They think that the soul is only a vital something, purely ethereal, of which no idea can be formed.

(3.) That it is the body alone which feels, sees, and hears.

(4.) That man is like a beast, with this difference only, that he can speak from thought.

(5.) That nature is all, and the first source from which all things proceed.

(6.) That man imbues and learns to think by an influx of interior nature and its order.

(7.) That there is no spiritual, and if it is, that it is a purer natural.

(8.) That man cannot enjoy any blessedness, if deprived of the delights of the love of glory, honor, or gain.

(9.) That conscience is only a disease of the mind, proceeding from the infirmity of the body and from not having success.

(10.) That the Divine love of the Lord is the love of glory.

(11.) That there is no providence, but that all things come to pass from one's own prudence and intelligence.

(12.) That honors and riches are real blessings which are given by God. Not to mention many other things of a similar nature. Such are the fallacies of the senses in spiritual things. Hence it may appear, that heavenly things cannot be comprehended by those who are merely natural and sensual. Those are merely natural and sensual whose internal spiritual man is shut, and whose natural only is open.