Revelation 12:7-9 describes an end-times angelic battle between Michael and 'his angels' and the devil and 'his angels.' From these and similar verses, it is clear that demons and fallen angels are synonymous. We know that hell was prepared for the devil and his angels, according to Matthew 25:41: “Then He will say to those on His left, ‘Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.’” Jesus, by using the possessive word his makes it clear that these angels belong to Satan. There is no doubt these fallen angels are now known as the demons.
So when Satan, who was once the angel Lucifer, rebelled against God and fell from heaven (Isaiah 14 Ezekiel 28), one third of the angelic host joined his insurrection (Revelation 12:3-4,9). When exactly God created angels is open for debate, but what is known for sure is that God created everything good because God, in His holiness, cannot create something sinful.
