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.

1692. What temptations, or the conflicts that constitute temptations, accomplish, scarcely anyone is able to know. They are the means by which evils and falsities are broken up and dispersed. They are also the means by which an abhorrence of these is produced, and a conscience is not only given but also strengthened, and man is accordingly regenerated. This is the reason why those who are being regenerated are led into conflicts and undergo temptations and why those who do not experience them during their earthly life do so in the next life, provided they are such as can be regenerated. And it is for these reasons also that the Lord's Church is called 'militant'. The Lord alone however underwent from His own strength or power the most cruel conflicts that constitute temptations, for He was beset by all the hells and constantly overcame them.

[2] It is also the Lord alone who fights in the temptation-conflicts of those who do undergo these. Man by his own power can accomplish nothing in the slightest against evil or hellish spirits, for they are so closely linked to the hells that if one were vanquished another would rush to take his place, and so on for ever. They are like the sea pressing against every part of a dike. If the dike were to develop a split or crack the sea would never cease to pour through and flood the land until nothing was left standing. It would be like this with man if the Lord alone did not bear up against the conflicts experienced by man in temptations.