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.

1450. 'He removed from there to the mountain on the east of Bethel' means the Lord's fourth state when He was a boy. This becomes clear from what comes before and after, thus also from the train of thought itself, namely this: The Lord had first of all to be endowed from infancy with the celestial things of love - the celestial things of love consisting in love towards Jehovah and love towards the neighbour, and in innocence itself present in those loves. From these, as from the very sources of life, flows every single thing, for all other things are simply derivatives. These celestial things are implanted in a person chiefly in the state of infancy through to childhood, and in fact independently of cognitions; for they flow in from the Lord and stir that person with affection before he knows what love is or what affection is, as the infant state shows, and after that the state of early childhood. Those things with man are the 'remnants' that have been referred to several times which are implanted by the Lord and stored away for use in his later life. For these matters, see 468, 530, 560, 561, 660, 661. Since the Lord was born as any other is born, He too was introduced according to order into celestial things, step by step from infancy on into childhood, and after that into cognitions. These things as experienced by the Lord are described in the present verse, and represented in those that follow by Abram's sojourning in Egypt.