TWH: The existence of demons is pretty much assumed throughout the Old and New Testaments. Justin Martyr believed that demons were the byproduct of an unholy union between angels and women; women conceived and begat demons. This is no doubt a connection to Genesis 6. There are problems here with what Justin argues. In fact, demons and fallen angels are synonymous. The resulting offspring of the unions in Genesis 6, which we will discuss later in this series, are physical beings, not spiritual. The demonic activity in the Gospels, for example, is all the work of fallen spiritual beings. The distinction that Justin Martyr creates is not sustainable.
In this regard, Origen is correct. He argues that demons are not evil beings who have always been evil. Instead, the name "demons" refers strictly to angels who chose to disobey their Creator. The Bible sheds some light on this event. Satan is described in Isaiah 14 and Ezekiel 28. God tells us in Ezekiel that Satan was in the Garden of Eden (Ezek. 28:13): "You were in Eden, the garden of God." He was clothed in beauty (Ezek. 28:13), anointed (Ezek. 28:14), blameless (Ezek. 28:15), wise (Ezek. 28:17), etc. But something happened with Satan. Ezekiel 28:15 tells us that there came a point in time that unrighteousness was found in this one, and he sinned (v. 16). What caused this change? How did Satan shift gears and move from blameless to sinful? How'd he go from not having a single trace of sin to having a multitude of iniquities (Ezek. 28:18)? Ezekiel 28:17 reads, "You heart was lifted up because of your beauty; you corrupted your wisdom by reason of your splendor." And God responded to this act of disobedience. We see in Ezekiel that God casted him to the ground (Ezek. 28:17).
Isaiah shows us five intents Satan possessed during this time. Each of them is marked by "I will." These are the things that he said in his heart. The first three "I will" statements are found in Isa. 14:13: (1) "I will ascend to heaven;" (2) "I will raise my throne above the stars (angels) of God;" (3) "I will sit on the mount of assembly in the recesses of the north." The second two "I will" statements are found in Isa. 14:14: (4) "I will ascend above the heights of the clouds;" and (5) "I will make myself like the Most High." Satan's intent is to overthrow the God of the universe, usurp everything that belongs to God, and install himself as the object of all creation's praise and worship.
It's not until we get to Revelation that we see the scope of this act of disobedience. The details are not provided. We get declarative statement that something happened, but not the intricacies of the entire event. What we do learn in Revelation is that Satan was not alone in being cast down to the earth. Revelation 9:1 says that Satan fell (note that he is referred to as a 'star') from the sky to the earth. Revelation 12:4 tells us that Satan "swept away a third of the stars of heaven." Revelation 12:9 tells us that "his angels were thrown down with him." One-third of the heavenly host participated in this rebellion against God. And they had aligned themselves no longer with the one who deserved their worship, but instead with the one who had led heaven's coup d'etat. One-third constitutes a large, large, large number of heavenly beings. That God refers to his angels as "sons" (e.g., Job 1) tells us just how much he values them. This was a devastating rebellion. When God judged them, he threw Satan and each participant out of his presence down to the earth. That's not to say they can no longer be in the presence of God. Demons, for example, are in the presence of Jesus repeatedly during his life and ministry. In fact, Jesus, Peter tells us, went and made proclamation to those spirits that were once disobedient in the days of Noah. These fallen beings can be in the presence of God, just like God can summon Satan to his presence in Job 1 and 2. But there relationship with him is forever destroyed. He has judged them and has issued a decree that he will ultimately destroy and punish them for their wickedness.