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.

2632. 'Abraham circumcised his son Isaac' means purification of the rational. This is clear from the meaning of 'being circumcised' as being purified, dealt with in 2039, and from the representation of 'Isaac' as the Divine Rational, dealt with in 2630.

[2] The Lord's first rational was born in the way it is with others, that is to say, by means of facts and cognitions. This has been stated already where Ishmael, who represents that rational, is the subject. Because it was born, as with others, by means of facts and cognitions, thus by the external way, which is that of the senses, this rational inevitably included many things that were worldly in origin, for it is from those things that the ideas which the rational possesses are acquired. This was even more so the case with the Lord on account of the heredity He had from the mother. It was those worldly things and this heredity which the Lord gradually cast out of His rational until it was such that it was able to receive the Divine, 2624, 2625. At this point the Lord's Divine Rational, represented by 'Isaac', was born, 2630, not indeed by the external way, which is that of the senses - as was the case with the previous rational - but by the internal way from the Divine itself, 2628, 2629. As this was not accomplished all at once but gradually, 1690, 2033, the rational was purified, and constantly so. It is this purification that is meant by 'Abraham circumcised his son, a son eight days old'. That the Lord gradually made His Rational Divine and purified it constantly is clear also in John,

Jesus said, Father, glorify Your name. A voice therefore came from heaven, I have both glorified it, and will glorify it again. John 12:28.

'Glorifying' is making Divine, see 1603, 1999.

[3] In the Ancient Church nothing else was represented and meant by circumcision than that a person should be purified from self-love and love of the world, which is also effected gradually and constantly, see 2039, 2046 (end), 2049, 2056, especially when a person has been born anew or regenerated. For at that time the Lord is flowing in by the internal way, that is, through the good that forms part of conscience; and He gradually and constantly separates the things which cling to that person both as a result of hereditary evil and of the evil of his own doing.