Revelation 9:13; Voice from the Four Horns of the Golden Altar

Revelation 9:13 And the sixth angel sounded, and I heard a voice from the four horns of the golden altar which is before God, 

The voice that speaks from the golden altar to the sixth angel blowing the trumpet is an answer to the prayers the saints in heaven and on earth have presented to God (6:9–10; 8:3–5).

Revelation 6:9-10 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? 

Revelation 8:3-5 And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense, that he should offer it with the prayers of all saints upon the golden altar which was before the throne. And the smoke of the incense, which came with the prayers of the saints, ascended up before God out of the angel’s hand. And the angel took the censer, and filled it with fire of the altar, and cast it into the earth: and there were voices, and thunderings, and lightnings, and an earthquake. 

None of the other angels with trumpets receives such a direct communication from the presence of God Himself.

“And I heard a voice from the horns of the golden altar which is before God.” Even though John mentions God’s name, he notes that the sound is coming from the altar, namely, the very presence of God. Thus, he avoids referring to God but reverently says that the voice originates from the golden altar. John used a similar method of indirect identification for Jesus when he called him the voice like a trumpet (1:10).

Luke 18:7-8a And shall not God avenge his own elect, which cry day and night unto him, though he bear long with them? I tell you that he will avenge them speedily. Amen.

 Kistemaker, S. J., & Hendriksen, W. (1953–2001). Exposition of the Book of Revelation (Vol. 20, p. 294). Grand Rapids: Baker Book House.