I'll copy here the dissertation I once wrote on this.
It's amazing to me how many UnBiblical ideas are casually believed even by many Fundamentalist Christians simply because Society has conditioned us to believe it.
The idea of a War in Heaven sometime before Adam's fall is a Gap Theory connected false Doctrine mainly popularized by Secular Western fiction.
When I say it's a Pagan idea it's because various Pagan mythologies have myths of various wars between the gods (Sometimes newer gods overthrowing older ones) or of a god being cast out. Greek mythology as the Titanarchy, the Gigantarchy, Zeus overthrowing Ophion, the brief usurpation of Typhon, and the casting out of Hephestos. Egyptian mythology has Amon's war with Apep and Set overthrowing Osiris to latter be overthrown by Horus. Norse mythology has the war of the Vanir and the Aseir. And so on.
The Bible only ever discuses the Fall of Satan is Prophetic books, Isaiah 14:12, Ezekiel 28:13, and Revelation 12. Only one isolated verse appears to place it in the past, Luke 18:10 "And I beheld Satan as lighting fell from Heaven". But God often uses past tense language in reference to future events, and Yeshua is God The Son made flesh, he came from Outside time, he's seen all of History already play out.
Job demonstrates that Satan still has free access to Heaven, and he's still serving that function in the New testament, hence why he's called the Accuser. The only difference now is that now believers have an Advocate in Yeshua. 1 John 2:1
The big problem with this false view to me is that it conflicts with the important fact that Adam's sin is the origin of Sin and Death. Roman 5:12, 1 Corinthians 15:21. Genesis 3 is the origin of Sin, it's not just the first Sin Adam or Eve committed, I believe it's the Serpent's first open act of rebellion against God as well. We don't need a Prequel Trilogy to explain how The Serpent became a sinner.
The Fallen Angels did not all fall at once. The first group to fall are the eons form Genesis 6, who are now imprisoned in the Abyss. Then after the Flood more began to fall. I suspect there may be some un-fallen now who will fall before the end.
Satan is also still the "Archon (Prince or Ruler) of the World" (John 12:31, 14:30, 16:11) as well as the "God of this World" (2 Corinthians 4:4). Some think the John verses mean he ceases serving as that at the Crucifixion, but the Corinthians verse shows he still is, and Ephesians also shows his servants still rule the World behind the scenes. Yeshua did purchase the Archonship with his Death and Resurrection, but he doesn't actually take it until the Millennium.
Isaiah 14 is a continuation of 13, it's still the same Prophecy. It affiliates Satan's final doom with the finale Doom of Babylon, which is yet future, (Revelation 17 and 18). But the exact timing I feel requires Revelation 12 to figure out.
Revelation 12 begins as a summery of all History, the Woman being Israel, the War in Heaven begins not only after The Man Child's birth (3 B.C.) but after the ascension (30 A.D.) and after the Woman/Israel has fled to her hiding place in the Wilderness (Petra/Edom, Isaiah 63), which as we know from Matthew 24 is triggered by the Abomination of Desolation. When it says he caused a Third of the stars to fall from Heaven, I believe that's referring specifically the Angels who fell in Genesis 6 who are now in Tartaros.
In other words it's the Middle of the Tribulation. Daniel 11 jumps from the Hellenistic age to the end times in verse 35. Verses 36 till the end of the chapter, I believe outline The Little Horn's activities during the 1st half of The Tribulation, ending with setting the stage for his counterfeit Death and Resurrection. So when the first verse of Chapter 12 says '"And at that time shall Michael stand up" I believe that corresponds directly to Revelation 12:7.
