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.

255. 'The seed of the woman' is used to mean faith in the Lord. This is clear from the meaning of 'woman' as the Church, the seed of which is nothing else than faith. It is from faith in the Lord that the Church has its being and gets its name. In Malachi faith is called 'God's seed',

Jehovah was a witness between you and the wife of your youth. And not one has done so, though he has a residue of the spirit. And is there one seeking God's seed? But take heed within your spirit, lest he act treacherously against the wife of your youth. Malachi 2:14-15.

Here 'wife of your youth' is the Ancient Church and the Most Ancient, whose seed or faith is the subject. In Isaiah,

I will pour out waters upon the thirsty land, and streams on the dry. I will pour out [My] Spirit upon your seed and My blessing upon your descendants. Isaiah 44:3.

This too refers to the Church. In Revelation,

The dragon was angry with the woman, and went off to make war with the rest of her seed, who kept the commandments of God and bear testimony to Jesus Christ. Revelation 12:17.

And in David,

I have made a covenant with My Chosen One, I have sworn to David My servant, I will establish your seed even for ever; and I will set up his seed in perpetuity, and his throne as the days of the heavens; his seed will be for ever, and his throne will be like the sun in front of Me. Psalms 89:7, 4, 29, 36.

Here David is used to mean the Lord, 'his throne' to mean His Kingdom, 'the sun' love, and 'seed' faith.