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.

52. 6. Jehovah himself - that is, the Lord - spoke the Word through prophets.

We read of the prophets that they had visions and that Jehovah talked with them. When they had visions, they were not focused on their bodies but on the spirit, in which state they saw things of a heavenly nature. When Jehovah talked with them, though, they were conscious of their bodies and heard Jehovah speaking.

We need to draw a clear distinction between these two states. In a visionary state, the eyes of their spirit were open and the eyes of their body were closed; and at such times they seemed to themselves to be taken from place to place while their bodies stayed where they were. Ezekiel, Zechariah, and Daniel were in this state at times, and so was John when he wrote the Book of Revelation. They were then said to be in a vision or in the spirit. In fact, Ezekiel says,

The spirit lifted me up and brought me back into Chaldea, to the captivity, in a vision from God, in the spirit of God. In this way the vision that I saw came over me. (Ezekiel 11:1, 24)

He says that the spirit lifted him up and that he heard an earthquake and other things behind him (Ezekiel 3:12, 14). He also said that the spirit lifted him up between earth and heaven and took him off into Jerusalem in visions from God, and he saw abominations (Ezekiel 8:3 and following).

That is why (again in a vision of God or in the spirit) Ezekiel saw the four beasts that were angel guardians (chapters 1; 10), and he saw a new earth and a new temple with the angel measuring them, as we are told in chapters 40-48. He says in chapter 40:2, that he was then in visions from God; and in chapter 43:5, he says that the spirit lifted him up at that time.

The same thing happened with Zechariah. There was an angel inwardly present with him when he saw a man riding among myrtle trees (Zechariah 1:8 and following); when he saw four horns and then a man with a measuring line in his hand (Zechariah 1:18; 2:1); when he saw Joshua the high priest (Zechariah 3:1 and following); when he saw a lampstand and two olive trees (Zechariah 4:1 and following); when he saw a flying scroll and a measuring basket (Zechariah 5:1, 6); and when he saw four chariots coming from between two mountains, along with horses (Zechariah 6:1 and following).

Daniel was in the same kind of state when he saw four beasts come up from the sea (Daniel 7:3) and when he saw battles between a ram and a goat (Daniel 8:1 and following).

We read in Daniel 7:1-2, 7, 13; 8:2; 10:1, 7-8 that he saw these things in visions. We read in Daniel 9:21 that he saw the angel Gabriel in a vision and talked with him.

Much the same happened with John when he wrote the Book of Revelation. He says that he was in the spirit on the Lord’s day (Revelation 1:10), that he was carried away in the spirit into the wilderness (17:3), to a high mountain in the spirit (21:10), that he saw horses in a vision (9:17), and elsewhere that he saw what he described, being therefore in the spirit or in a vision (1:2; 4:1; 5:1; 6:1; and in the particular chapters that follow).