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.

8067. 'And as a memorial between your eyes' means that it shall be constantly in their understanding. This is clear from the meaning of 'memorial' too as constant recollection, the word 'memorial' being used because it has reference in the Word to the understanding, whereas 'sign' has reference to the will; and from the meaning of 'the eyes' as the understanding, dealt with in 2701, 3820, 4403-4421, 4523-4534. This is why 'as a memorial between the eyes' means that it shall be constantly in their understanding, that is, their thinking.

Something brief will be said to show how one should understand the explanation that it shall be constantly in the understanding and constantly in the will. The things which have imprinted themselves on a person through faith and charity, that is, which he fully believes and loves, are constantly in his thought and will. He thinks them and wills them even when he contemplates and deals with other matters, and imagines that those things are not present in his mind at that time. For they are indeed there among other things which determine the character of his mind. The truth of this is plainly evident from the spiritual sphere surrounding a spirit or angel; for when the spirit or angel approaches, that sphere tells one immediately about his faith and about his charity, and about much more that he has at heart, even though he is not thinking about them at the time. Such are the things that constitute the life of every mind and remain constantly there. These considerations could be illustrated by very many details to be observed in a person, such as his reflections on different things, his affections, the ways to act that have been ingrained in him since early childhood, and the like, which are continually present and hold sway although he does not consciously think about them at all. It is similar with love to the neighbour, love to God, and a love of goodness and truth, and with faith. People with those kinds of love and with faith will and think them constantly; for such love and faith reside within these people, and when they do so they are said to be reigning throughout, see 6159, 6571, 7648.