1. After this I heard something like the loud voice of a vast multitude in heaven, saying, Hallelujah! Salvation, glory, and power belong to our God,
A vast multitude is most likely the angelic beings in heaven who are witnessing the destruction of the false religion/Babylon, see Revelation 5:11
The praises given to God for the judgment is because it is a righteous act.
2 because his judgments are true and righteous, because he has judged the notorious prostitute who corrupted the earth with her sexual immorality; and he has avenged the blood of his servants that was on her hands.
Sexual immorality is not physical but spiritual. This false system convinced the masses into following a false Christ even though the people thought that they were indeed following Jesus but they were deceived.
In Scripture, it is said that there is a false Jesus, a false Gospel, and a false spirit which we should be very careful of lest we fall into Satan’s trap of deception.
2 Corinthians 11:4
For if a person comes and preaches another Jesus, whom we did not preach, or you receive a different spirit, which you had not received, or a different gospel, which you had not accepted, you put up with it splendidly!
Those who have persecuted Christians will be judged by Christ and will be found guilty.
3 A second time they said, Hallelujah! Her smoke ascends forever and ever!
Hallelujah is a compound word that has halal and Yah which is of Hebrew origin, see Gk. Strong’s# 239. The word literally means, “praise Yahweh.”
The angels in heaven praised the Lord a second time because evil was punished.
It is a reminder that no one will ever get away with the evil they have done unless they repent.
4 Then the twenty-four elders and the four living creatures fell down and worshiped God, who is seated on the throne, saying, Amen! Hallelujah!
For further study on the 24 elders, see Revelation 4:4
The grand display of God’s judgments and His righteous anger moved the 24 elders and the four living creatures to worship the Living God in being in awe of Him and His power.
5 A voice came from the throne, saying, Praise our God, all his servants, and the ones who fear him, both small and great!
There’s a call to the saints to praise the Lord because all the evils that were done to them, God is bringing retribution on those evil men that have done them harm.