Revelation 9:13

"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 filled up. They do not overpass their bounds, because at the last they shall come with Antichrist.

When it says that the first woe has passed and the trumpet of the sixth angel has sounded, it announces the final preaching, that of the sixth age.

In the altar that is in the sight of God we are to understand the church. In the time of the last persecution she will despise the words and commands of that most inhumane of kings and will separate from those who have submitted to him.

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.

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.

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. 33:16] The horns of the altar are holy preachers, and by this one horn we understand the leader of preachers, that is Christ. He tells the sixth angel sounding the trumpet, that is, he proclaims to the Church, to loose the four angels, because he has granted it the power to bind and loose in heaven and on earth, according to this: Whatsoever thou shalt bind, etc. [Matt. 16:19] In the four angels we recognize the entire body of the reprobates, which consists of evil spirits and wicked men, of course separate in nature, but united in iniquity. Indeed the number four is frequently used to signify totality, whence it is said that in the end of the world the elect will be taken from the four winds of the sky by angels. [Matt. 24:31 and Mark 13:27] By the Euphrates, which is often mentioned with a negative meaning, is represented the worldly kingdom's power, hostile to Christ and the Church; and the old enemy is said to be bound in this power with his body because he is not permitted to do as much harm as he wants. So let us take Loose the four angels who are bound in the great river Euphrates as if the Church were told, "Loose both men in angels, and angels in men, and bear in mind how much you ought to endure the hardships of persecution with equanimity, since your enemies are not able to inflict them on you except by your own judgment, which is in your Head, which proclaims for you and in you, All power is given to me in heaven and in earth." [Matt. 28:18]