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.

8806. 'And sanctified the people' means covering over their interiors in order that those people may appear in the holiness of faith. This is clear from the explanation above in 8788. That paragraph however showed what sanctification among the Israelite people was, namely a covering over of their interiors in order that those people might appear outwardly in holiness when they were involved in representative worship. And unless their interiors had been covered over then, foul thoughts and evil affections would have not merely defiled the holiness of their worship but also completely destroyed it, so completely that nothing celestial or spiritual, represented in externals, could have been perceived from it in heaven.

[2] But since 'the children of Israel' is used here to mean those belonging to the spiritual Church, something brief will be stated about how to understand sanctification among those people. Those belonging to the spiritual Church, those who lead a life of truth, and go on to lead a life of good, are withheld from evils and maintained in good by the Lord. The good they have from the Lord is the holiness with them. Consequently in the measure that they receive good from the Lord they are holy; and the measure of good they receive from the Lord, that is, the measure in which they are holy, is determined by how far they lead a life of good in keeping with the genuine truths of faith, and by how far at that time they believe that all the good they think and do then begins in the Lord. When they do these things the evils present with them are separated so that they do not at all appear, especially the evils they have derived by heredity from their parents, which are foul, because they have been accumulated from many ages back by successive generations of parents and accordingly heaped up in offspring. These are the interiors which with those belonging to the spiritual Church have been hidden away and so to speak covered over, 966, 1667, 2307, 2308, 3701, 4317, 8550. So it is that the entire will they receive by heredity has been corrupted. As a consequence of this they have a new will which the Lord forms in the understanding part of the mind by means of the truths of faith. This explains why they are led by means of truths to good while they are being regenerated, and after that, when they have been regenerated, from good into the truths of good. This shows plainly how their new will is created by the Lord. Regarding its formation in the understanding part of the mind with those belonging to the spiritual Church, see 863, 875, 895, 927, 1023, 1043, 1044, 1555, 2256, 4328, 4493, 5113, 6125.