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.

2777. 'On one of the mountains' means Divine love. This is clear from the meaning of 'a mountain' as love, dealt with in 795, 796, 1430, here Divine love since it has reference to the Lord. For the nature of that love, see 1690, 1601 (end), 1789, 1812, 1820, 2077, 2253, 2500, 2572. As it was Divine love from which the Lord fought and overcame in temptations, and from which He sanctified and glorified Himself, Abraham is in this verse told to offer Isaac as a burnt offering 'on one of the mountains' in the land of Moriah. Light is shed on this representative act by the fact that David built an altar, and Solomon the Temple, on Mount Moriah, 2775. For the altar on which burnt offerings and sacrifices were offered, and later the Temple, was the chief representative of the Lord. The fact that the altar was the chief representative of Him may be seen in 921 and is clear in David,

They will bring me to Your holy mountain and to Your dwellings. Then I will go in to the altar or God, to God my exceeding joy. Psalms 43:3-4.

And that 'the Temple' was representative of Him is clear in John,

Jesus said, Destroy this temple and in three days I will raise it up again. He was speaking of the temple of His body. John 2:19, 21.