Are demons fallen angels?

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. 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). There is no doubt these fallen angels are now known as the demons.

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. 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.

Some reject the idea that the demons are the fallen angels due to the fact that Jude verse 6 declares the angels who sinned to be "bound with everlasting chains." However, it is clear that not all of the angels who sinned are "bound," as Satan is still free (1 Peter 5:8). Why would God imprison the rest of the fallen angels, but allow the leader of the rebellion to remain free? It seems that Jude verse 6 is referring to God confining the fallen angels who rebelled in an additional way, likely the "sons of God" incident in Genesis chapter 6.

The most common alternate explanation for the origin of the demons is that when the Nephilim of Genesis 6 were destroyed in the Flood, their disembodied souls became the demons. While the Bible does not specifically say what happened to the souls of the Nephilim when they were killed, it is unlikely that God would destroy the Nephilim in the Flood only to allow their souls to cause even greater evil as the demons. The most biblically consistent explanation for the origin of the demons is that they are the fallen angels, the angels who rebelled against God with Satan.

"Is Lucifer Satan? Does the fall of Lucifer describe Satan?"

Answer: There is no verse in the Bible that says, “Lucifer is Satan,” but an examination of several passages reveals that Lucifer can be none other than Satan. The fall of Lucifer described in Isaiah 14:12 is likely the same that Jesus referred to in Luke 10:18: "I saw Satan fall like lightning from heaven.” A similar fall is depicted in Ezekiel 28.

Isaiah 14:12–18 describes the fall from heaven of one called “Lucifer,” a name that means “morning star,” “son of the dawn,” “Day Star / Daystar,” or “shining star.” The description of the one referred to shows us it can be none other than Satan. We know from Jesus’ own words in Luke 10 that Satan fell from heaven. So, when Isaiah refers to Lucifer (In Hebrew, helel) being cast down to earth (Isaiah 14:12), it can be none other than Satan. The reason for his fall is found in verses 13 and 14: “You said in your heart, ‘I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of the sacred mountain. I will ascend above the tops of the clouds; I will make myself like the Most High.’” This has always been Satan’s desire—to be God—and it is the very temptation he used in the garden of Eden to get Eve to disobey God: “You shall be as God” (Genesis 3:5).

Ezekiel 28 is another passage thought to refer to Lucifer/Satan. Although it begins with Ezekiel being commanded by God to “take up a lament concerning the king of Tyre” (v. 12), an evil idolatrous king, it soon becomes clear that the passage is referring as well to the power behind that king—Satan. Verse 13 says he was “in Eden, the garden of God.” Clearly, the king of Tyre was never in Eden. Verse 14 says, “You were anointed as a guardian cherub, for so I ordained you.” Apparently, Lucifer/Satan had a position of guardian angel in heaven “among the fiery stones,” thought to be the shining precious jewels that are seen in other descriptions of heaven (Exodus 24:10; Revelation 21:18–21). Since the king of Tyre was never in heaven, either, this can only be describing Lucifer. The rest of the passage describes the reason he was cast out of heaven. Because of his beauty, his heart became proud and his wisdom was corrupted (Ezekiel 28:17). Pride in his perfection, wisdom, and beauty (v. 12) became the source of his downfall, and God threw him to the earth (v. 17). This was witnessed by the Lord Jesus in heaven before His incarnation (Luke 10:18).

To summarize, Lucifer was cast out of heaven for his sin of pride and his desire to be God. Jesus referred to seeing Satan being cast out of heaven. Therefore, we can conclude that Lucifer and Satan are one and the same.