When the sixth trumpet sounds, an angel next to the golden altar in the heavenly temple of God commands the angel of the sixth
trumpet to release of four bound angels at the Euphrates River.
In contrast to the fifth trumpet which initiates an angel to go unlock the abyss
(Rev 9:1), there is no mention of how the angel with the sixth trumpet releases the four
bound angels at the Euphrates River.
While the location of the four bound angels is uncertain, it is likely not the abyss.
When the Bible speaks of confined or bound angels, it is always in the context of fallen evil angels
(Luke 8:26-31; Matt 8:28-32;
2 Peter 2:4; Jude 1:6-7).
Just as with the fifth trumpet, God uses evil demonic instruments to carry out His judgment. Here the four bound fallen evil
angels serve a divine purpose.
God's scheduled the release of the four bound angels to commence at the exact hour of a precise day of a certain
month and of a specific year (Rev 9:15).
The four bound angels were limited to kill 1/3 of the population at that time. In consideration of the fourth seal,
in which 25% of the population was killed, the death of 1/3 of the surviving 75% leaves at most 50% of the original population before
the tribulation began.
How human beings are killed is worth noting:
A third of mankind was killed by these three plagues, by the fire and the smoke and the brimstone which
proceeded out of their mouths. For the power of the horses is in their mouths and in their tails; for their tails are like serpents
and have heads, and with them they do harm. (Rev 9:18-19)
The riders did not kill anyone.
The apostle John observes that after the four bound angels complete their task of carrying out God's judgment, the surviving human
beings, at most 50% of the original population before the tribulation, do not recognize that the plagues confirm the existence of God
and are unrepentant.
The rest of mankind, who were not killed by these plagues, did not repent of the works of their hands, so
as not to worship demons, and the idols of gold and of silver and of brass and of stone and of wood, which can neither see nor hear
nor walk; and they did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts.
As in the case of Pharaoh, when a heart is so hard, including those who profess a faith but in reality lack one,
then God uses "plagues" to exact His judgment of death.
If you are not careful to observe all the words of this law which are written in this book, to fear this
honored and awesome name, the Lord your God, then the Lord will bring extraordinary plagues on you and your descendants, even
severe and lasting plagues, and miserable and chronic sicknesses. He will bring back on you all the diseases of Egypt of which
you were afraid, and they will cling to you. Also every sickness and every plague which, not written in the book of this law,
the Lord will bring on you until you are destroyed. (Deut 28:58-61)