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.

1150. That 'the sons of Japheth' means those who possessed external worship corresponding to internal has been stated already. It is called external worship corresponding to internal when the worship has what is essential within it. And that which is essential is adoration of the Lord from the heart, which in no sense exists unless charity, or love towards the neighbour, is present. It is in charity or love towards the neighbour that the Lord is present, in which case it is possible for Him to be adored from the heart. Thus it is from the Lord that adoration comes, for the Lord makes all adoration possible and gives it all its being. It follows therefore that as is a person's charity, so is his adoration or worship. All worship is adoration, for adoration of the Lord must be in it for it to be worship. The sons of Japheth, that is, the nations and peoples called the sons of Japheth, lived with one another in mutual charity, in friendship, in civility, and in simplicity, and consequently the Lord was present in their worship. For when the Lord is present in external worship internal worship is in that case within the external, that is, there is external worship corresponding to internal. In former times very many nations were such, and nations also exist at the present day who focus worship on external things without knowing what the internal is. And if they do know they give no thought to those matters. If those people acknowledge the Lord and love the neighbour, the Lord is in their worship and they are 'sons of Japheth'. But if they deny the Lord, love only themselves and do not care about the neighbour, worse still if they hate him, then their worship is external separated from internal, and they are 'sons of Canaan' or Canaanites.