Fall of Satan I did my college thesis on the Fall of Satan, because it is important in establishing the Lordship of Christ over the world in the Kingdom of God! You picked a good topic.
First, the alleged "Gap theory" underwritten by the Scofield Reference Bible, is an imaginary theory promoted by the Dispensational sect. It was not traditional orthodoxy, but promoted by a young 28 year old man in England in 1928. (See Tim LaHaye, No Fear of the Storm; Scofield Reference Bible, 1919.)
Old Testament Then notice that in the Old Testament, Satan is pictured as having entrance into the Presence of God on several occasions: He is maligning Job, he is opposing the High Priest during the exile, etc. And in the New Testament, he even dares to tempt the Incarnated God, Jesus!
But with the spiritual ministry of Jesus, and the power of the Holy Spirit, there is a change:
The seventy-two (disciples) returned with joy and said, "Lord, even the demons submit to us in your Name!"
Jesus replied, "I was seeing Satan fall like lightning from heaven. I have given you authority to trample on snakes and scorpions and to overcome all the power of the enemy... (Luke 10:17-19)
The tense of the verb here showed that Jesus was having a spiritual vision of a real occurence. Satan was being cast out of his privileged position, such that now even disciples of Christ had authority over him! This power was reiterated in Mark 16:15-18. (Even if this passage was not part of the original manuscript, it showed the intent, practice, and interpretation of the Early Church leaders.)
Jesus in Control The Bible clearly describes Jesus as having all authority in heaven and on earth (Matthew 28:18) and raised from the dead and seated at the right hand of the heavenly realm, far above all rule and authority, power, and dominion (Ephesians 1:20-21). There is room for only one power in heaven...and it isn't Satanic!
Misinterpretations Many are misled by modern interpreters concerning some O.T. passages. For example, Isaiah 14:12-16. Some say this fall of Lucifer referred to Satan. But the Bible itself interpreted this fall as that of a "man", the king of Babylon! Anyone who has studied ancient Eastern cultural languages concerning royalty realized their use of lofty, divine, exalted imagery. This passage is expected for a description of that king...and his downfall and conquering. It is not a reference to Satan. Modern Western readers mistake the verbage. Reading books on ancient culture, and archaeology, and ancient religions, would clear this up.
Ezekiel 28:1-10 is another passage of scripture that is misunderstood. The description of someone "who sits on the throne of god" and who "thinks he is as wise as a god," misleads modern readers into thinking this is Satan.
But here again, the scriptures interpret this for us. You are a man, and not a god and Son of man, say to the ruler of Tyre. Also, I am going to bring foreigners against you, the most ruthless of nations. There is no way these verses can be construed as referring to Satan. Much of this was propagated by the Scofield Bible, but it misleads bible students.
Satan Now So where is Satan now? Paul wrote: The god of this age has blinded the minds of those who believe not... (2 Corinthians 4:4) The Apostle also stated that we are to put on the full armor of God so we can withstand the wiles of the Devil, so this shows he is active on earth (Ephesians 6:11). He still has power to tempt, deceive, oppress (possess?) people on earth. But he does not have power to control any saint of God. And ministry in Jesus's Name brings deliverance to those bound or oppressed by him.
Conclusion Satan did not fall before Creation. There is no verse stating that. The Bible showed him having access to the throneroom of God throughout history. But the appearance of Jesus as Savior and Deliverer changed all that. Jesus said He saw Satan fall when the Gospel of the Kingdom was proclaimed by the disciples! This was a new era with a new absolute Ruler in the Heavens! Christ enthroned, Satan demoted!
Jesus Christ, who has gone into Heaven and is at God's right hand, with angels, authorities, and powers in submission to Him (1 Peter 3:22)