The term “spirits” repeats the “unclean spirits” of Revelation 16:13. John now adds as explanation that these spirits are (literally) of demons.
Demons are angels who joined Satan in his rebellion in the beginning (2 Peter 2:4; Jude 1:6). Through satanic deception planted in people’s minds by demonic spirits, people across the centuries have worshipped and trusted idols instead of the living God. That is also why the term “demons” appears in the Old Testament to describe idols (Leviticus 17:7; Deuteronomy 32:17; Psalm 106:37–38). Those idols were not real but were products of Satan’s deception (Psalm 96:5; 1 John 4:1) and ultimately stand-ins for Satan himself (1 Corinthians 10:20). These demons are destined for destruction (Matthew 8:29; Matthew 25:41). Given that the dragon is Satan and Satan is the deceiver of the whole world (Revelation 12:9), we may expect “demonic spirits” to continue their deceptive work anywhere and anytime throughout the new dispensation.
14 For they are demonic spirits, performing signs, who go abroad to the kings of the whole world, to assemble them for battle on the great day of God the Almighty.