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.

789. Moreover, the rest of the Prophets have predicted in many passages what this church will be like; only a few will be quoted. In Zechariah:

There shall be one day, which is known to Jehovah, neither day nor night, because around eveningtime there shall be light. On that day living waters shall come forth from Jerusalem; and Jehovah shall become king over the whole earth. On that day Jehovah shall be one, and His name one, Zechariah 14:7-9.

In Joel:

It will happen on that day that the mountains will drip new wine, the hills will flow with milk; and Jerusalem will stay fixed from and one generation to another, Joel 3:17-21.

In Jeremiah:

At that time they will call Jerusalem Jehovah's throne, and all nations will be gathered to Jerusalem for Jehovah's name's sake, and they will no longer follow the promptings of their evil hearts, Jeremiah 3:17; Revelation 21:24, 26.

In Isaiah:

Let your eyes behold Jerusalem, a tranquil dwelling, a tent which will not be blown away. Its pegs will never be removed, and its ropes will not be pulled up, Isaiah 33:20.

[2] In these passages Jerusalem means the new Holy Jerusalem described in Revelation (chapter 21); and this means the new church. In Isaiah again:

A shoot will go forth from the trunk of Jesse, and righteousness will be the girdle around his loins, and truth the belt around his thighs. Therefore the wolf will stay with the lamb, and the leopard with the kid, the calf and the young lion and the fatling together, and a small boy will lead them. The heifer and the bear will feed, their offspring will lie down together. The sucking child will play on the hole of the viper, and the weaned child will thrust his hand on the den of the basilisk. They will not do evil nor corrupt themselves in all the mountain of my holiness. For the earth will be full of the knowledge of Jehovah. It will happen on that day, the nations will seek the root of Jesse, which stands as a signal to the peoples, and peace will be his glory, Isaiah 11:1, 5-10.

It is well known that such events have not yet happened in the churches, least of all in the last of them. In Jeremiah:

Behold, the days are coming when I will strike a new covenant. This is the covenant: I will set my law in their midst, and I will write it on their hearts; and I will become their God and they shall become my people. All shall know me from the least to the greatest of them, Jeremiah 31:31-34; Revelation 21:3.

[3] It is also well known that these events did not take place in the previous churches. The reason is that they did not approach the visible God, whom all are to know. It is also because He is the Word, that is, the Law, which He will set in their midst and write upon their hearts. In Isaiah:

For Jerusalem's sake I will not rest, until her righteousness goes forth like a beam of light, and her salvation burns like a lamp. You will be called by a new name, which the mouth of Jehovah will utter. And you will be a crown of beauty and a diadem of kingship in the hand of your God. Jehovah will be pleased in you, and your land will be married. Behold, your salvation is coming, behold, his reward is with him; and they will call them the people of holiness, the redeemed of Jehovah, and you will be called a city sought after and not abandoned, Isaiah 62:1-4, 11-12.