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.

'The six days' represent the various consecutive states in a person's regeneration. They are, in general, as follows: The first state is the state which precedes both the state from earliest childhood onwards and which existed immediately before regeneration. This state is called 'a void, emptiness, and thick darkness.' And the first motion, which is the Lord's mercy, is 'the Spirit of God hovering over the face of the waters.' The second state is when a distinction is made between the things that belong to the Lord and those that are a person's own. The things which are the Lord's are called in the Word 'remnants,' and here they are mainly the thoughts about faith which a person has learned since he was a small child. These are stored away and do not come out into the open until he reaches this state. Nowadays, this state rarely occurs without temptation, misfortune, and sorrow, which lead to the inactivity and, so to speak, the death of bodily and worldly concerns -- the things which are a person's own. In this way, what belongs to the external self is segregated from what belongs to the internal. Within the internal are the remnants, stored away by the Lord until this time and for this purpose. The third state is one of repentance, a state in which he speaks piously and devoutly from the internal self and does good things, such as charitable acts which are still soulless because he imagines that they originate in himself. They are called 'a tender plant,' then 'a seed-bearing plant,' and finally 'a fruit tree.' The fourth state is when the person is moved by love and enlightened by faith. Prior to this, he would utter pious words and do good deeds, but he did so from a state of temptation and anguish, and not from faith and charity. However, these are now kindled in his internal self, and are called 'the two great lights.' The fifth state is when the person speaks from faith, and in so doing, he confirms himself in truth and good. What he does at this point have life and are called 'the fish of the sea and the birds of the air.' The sixth state is when the person speaks truth and performs good deeds from faith and so from love. What he does at this point are 'living creatures' and 'beasts.' At this point, he starts to act from faith and simultaneously from love, and he becomes a spiritual person, which is called 'an image.' The spiritual life of this person finds its delight in, and is sustained by the things which are associated with thoughts about faith and charitable acts, which are called 'his food.' His natural life finds its delight in, and is sustained by things which relate to the body and the senses. These things give rise to conflict until love rules and he becomes a celestial person. People who are regenerating do not all arrive at this final state, but some, and the most in the present day only to the first, some only to the second, some to the third, fourth, and fifth, few to the sixth, and hardly anyone to the seventh.

(Odkazy: Arcana Coelestia 6-13, Genesis 1)