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.

10123. 'And you shall cleanse from sin on the altar' means purification from evils in heaven and in the Church. This is clear from the meaning of 'cleansing' as purifying; from the meaning of 'sin' as evil, for all evil that a person is guilty of is called sin; and from the meaning of 'the altar' as that which is representative of the Lord in regard to Divine Good and of the worship of Him, dealt with in 9714, 9964, at this point in respect of Divine Good in heaven and in the Church. The subject is still the Lord's influx, presence, and reception there, and for this reason 'the altar' also means heaven and the Church in respect of reception of Divine Good from the Lord there. For that which is Divine and the Lord's is what makes heaven and the Church; this is so because the Lord dwells there in what is His and not in what is a person's own. This too explains why 'the altar' also means the actual person in whom heaven is present or in whom the Church is present, and so in whom the Lord is present; and in the abstract sense, in which no actual person is envisaged, 'the altar' means the Good itself which comes from the Lord and is present with the angels of heaven and with members of the Church.

[2] The word 'altar' is used with these meanings in John,

I was given a reed like a rod, and the angel stood and said to me, Measure the temple of God, and the altar, and those who worship in it. Revelation 11:1.

Here 'the temple of God, and the altar' are heaven and the Church, 'the temple' being the spiritual Church and 'the altar' the celestial Church. 'Measuring' means recognizing the amount and the essential nature of truth and good; and this is why it says not only 'Measure the temple, and the altar' but also 'those who worship in it'. For the meaning of 'the temple' as the spiritual Church, see 3720; and for that of 'measuring' as recognizing the state of affairs as regards truth and good, 9603. In the same book,

I heard another angel from the altar saying, O Lord God Almighty, true and righteous are Your judgements. Revelation 16:7.

'From the altar' means from the inmost heaven, where celestial good reigns, celestial good being the good of love to the Lord. In Jeremiah,

The Lord has abandoned His altar, He has abominated His sanctuary. Lamentations 2:7.

'Abandoning the altar and the sanctuary' refers to everything of the Church, 'the altar' being everything of the Church as regards good, and 'the sanctuary' everything of the Church as regards truth.