Oecumenius

When "the angel sounded", "I heard a voice" coming "from the horns of the altar". "The horns of the altar", he says, are those that stand out among the angels and surpass the others. He calls the altar "golden" because of its honor and divine nature, made from precious material known to us, depicting a marvelous altar. Just as we understood "the horns of the altar" to represent the rulers of the angels, so the altar itself symbolizes all the ministering spirits; because it offers a spiritual [λογικήν] sacrifice to God.

Commentary on Revelation ↗

Bede

And the sixth angel sounded his trumpet, etc. The sixth angel represents the preachers of the final battle, who, at the prompting of the Gospel, expose the frauds of the Antichrist. The horns of the golden altar indeed represent the preeminent Gospels of the Church.

Commentary on Revelation ↗

Alcuin of York

After these things the sixth angel also sounded the trumpet: and I heard the voice of one of the horns of the golden altar, which is before the eyes of God, saying to the sixth angel, who had the trumpet: Loose the four angels, who are bound in the great river Euphrates. The altar is the Church, and it is golden because it is illuminated by heavenly wisdom. Note also that the altar is before the eyes of God because the eyes of the Lord are upon the just. [Ps. …

Commentary On Revelation ↗

Victorinus of Pettau

"And I heard a voice from the four horns of the golden altar which is in the presence of God, saying to the sixth angel which had the trumpet, Loose the four angels." That is, the four corners of the earth which hold the four winds. "Which are bound in the great river Euphrates." By the corners of the earth, or the four winds across the river Euphrates, are meant four nations, because to every nation is sent an angel; as said the law, "He determined them by the number of the angels of God," until the number of the saints should be …

Commentary on the Apocalypse of the Blessed John ↗