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.

40. We can see that the Lord is called the Holy One of Israel from the following passages.

The angel said to Mary, “The Holy One that is born from you will be called the Son of God.” (Luke 1:35)

I saw in visions, and behold, a Watcher, a Holy One, coming down from heaven. (Daniel 4:13, 23)

God will come from Teman and the Holy One from Mount Paran. (Habakkuk 3:3)

[Thus says Jehovah, the Holy One of Israel, and Israel’s Maker.] (Isaiah 45:11)

Thus says Jehovah, the Redeemer of Israel, Israel’s Holy One. (Isaiah 49:7)

I am Jehovah your God, the Holy One of Israel, your Savior. (Isaiah 43:1, 3)

As for our Redeemer, Jehovah Sabaoth is his name, the Holy One of Israel. (Isaiah 47:4)

Thus says Jehovah your Redeemer, the Holy One of Israel. (Isaiah 43:14; 48:17)

Jehovah Sabaoth is his name, and your Redeemer, the Holy One of Israel. (Isaiah 54:5)

They tested God and the Holy One of Israel. (Psalms 78:41)

They have abandoned Jehovah and have angered the Holy One of Israel. (Isaiah 1:4)

They said, “Make the Holy One of Israel cease from our presence.” Therefore thus said the Holy One of Israel... (Isaiah 30:11-12)

... those who say, “Let him hasten his work so that we may see; let the counsel of the Holy One of Israel draw near and arrive.” (Isaiah 5:19)

On that day they will rely on Jehovah, the Holy One of Israel, in truth. (Isaiah 10:20)

Shout and rejoice, daughter of Zion, because the Holy One of Israel is great in your midst. (Isaiah 12:6)

The God of Israel has said, “On that day people will look back to their Maker, and their eyes will look toward the Holy One of Israel.” (Isaiah 17:7)

The meek will increase their joy in Jehovah, and the poor of the people will rejoice in the Holy One of Israel. (Isaiah 29:19; 41:16)

Nations will run to you because of Jehovah your God and because of the Holy One of Israel. (Isaiah 55:5)

The islands will trust in me to bring your children from afar for the name of Jehovah Sabaoth and the Holy One of Israel. (Isaiah 60:9)

Their land is full of sin against the Holy One of Israel. (Jeremiah 51:5)

There are many other instances elsewhere.

The Holy One of Israel means the Lord in his divine human nature, since the angel Gabriel said to Mary, “The Holy One that is born from you will be called the Son of God” (Luke 1:35). As for Jehovah and the Holy One of Israel being one and the same even though they are given different names, this is quite clear from the passages just cited where it says that Jehovah is the Holy One of Israel.