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.

1298. 'They had brick for stone' means that they had falsity in place of truth. This is clear from the meaning of 'brick' as falsity, dealt with just above, and also from the meaning of 'stone' in the broad sense as truth, dealt with already in 643. The reason 'stones' meant truth was that the most ancient people used to mark out boundaries by means of stones and raise up stones to testify that something was so, that is, was the truth. This is clear from the stone which Jacob set up as a pillar, Genesis 28:22; 35:14; from the pillar of stones placed between Laban and Jacob, Genesis 31:46-47, 52; and from the altar which the children of Reuben, Gad, and Manasseh erected beside the Jordan as an altar of witness, Joshua 22:10, 28, 34. Consequently 'stones' in the Word means truths, so much so that not only the stones of the altar but also the precious stones in the shoulder-pieces of Aaron's ephod and in the breastplate of judgement meant the holy truths of love.

[2] Regarding the altar, when sacrificial worship on altars was introduced, an altar in that case meant representative worship of the Lord in general. 'The stones' themselves however meant the holy truths belonging to that worship. This was why it was commanded that the altar had to be built of whole and not of hewn stones, and why it was forbidden to use any iron tool on them, Deuteronomy 27:5-7; Joshua 8:31. The reason was that hewn stones, and those on which an iron tool had been used, meant artificialities and thus fabrications in worship. That is to say, they meant things that derive from the proprium, or from the inventions of man's own thought and heart, which was to profane worship, as is clearly stated in Exodus 20:25. For the same reason no tool of iron was used on the stones of the Temple, 1 Kings 6:7.

[3] That the precious stones set in the shoulder-pieces of Aaron's ephod and in the breastplate of judgement in a similar way meant holy truths has been shown already in 114. This is clear also in Isaiah,

Behold, I will set your stones in carbuncle and lay your foundations in sapphires; and I will make your suns (windows) of ruby, and your gates into precious stones, and all your border into pleasant stones And all your sons will be taught by Jehovah, and great will be the peace of your sons. Isaiah 54:11-13

The stones mentioned here stand for holy truths, and this is why it is said that 'all your sons will be taught by Jehovah'. It is also the reason why it is said in John that the foundations of the wall of the city, holy Jerusalem, were adorned with every kind of precious stone, which are each mentioned by name, Revelation 21:19-20. 'The holy Jerusalem' stands for the Lord's kingdom in heaven and on earth, the foundations of which kingdom are holy truths. Holy truths were similarly meant by the tables of stone on which the commandments of the Law, or Ten Commandments, were written. This was why they were made of stone or had a stone base, concerning which see Exodus 24:12; 31:18; 34:1; Deuteronomy 5:22; 10:1; for the commandments themselves are nothing else than truths of faith.

[4] Now because stones in ancient times meant truths, and because later on when worship on pillars, on altars, and in the Temple began, pillars, altars, and the Temple meant holy truths, the Lord also is therefore called 'a Stone': In Moses,

The Mighty One of Jacob - from there is the Shepherd, the Stone of Israel. Genesis 49:24.

In Isaiah,

The Lord Jehovih said, I am laying in Zion for a foundation a Stone, a tested Corner-Stone, precious, of sure foundation. Isaiah 28:16.

In David,

The Stone which the builders rejected has become the head of the corner Psalms 118:22.

The same is meant in Daniel 2:34-35, 45, by the stone cut out of the rock which smashed Nebuchadnezzar's statue to pieces.

[5] That 'stones' means truths is clear in Isaiah,

By this the iniquity of Jacob will be expiated, and this will be the full fruit to remove his sin, when He makes all the stones of the altar like chalk-stones scattered about. Isaiah 27:9.

'The stones of the altar' stands for truths in worship that have been dissipated. In the same prophet,

Make level the way of the people; level out, level out the highway; gather out the stones. Isaiah 62:10.

'The way' and 'the stones' stand for truths. In Jeremiah,

I am against you, O destroying mountain. I will roll you down from the rocks and I will make you into a mountain of burning. And they will not take from you a stone for a corner, nor a stone for foundations. Jeremiah 51:25-26.

This refers to Babel. 'A mountain of burning' is self-love. 'Taking no stone from it' means that there is no truth from this source.