At one time, during the First Earth Age, when we were all spiritual beings that most people call angels, and all was peaceful (Job 38:7), Satan was the most beautiful and powerful of God’s angels; he was created to be the cherub that guarded the Mercy Seat. However, due to the fact that God created all of His children with free will, Satan developed pride and arrogance, and decided he was worthy of being worshiped, and not only that, but he decided that he was more powerful than God. So, because of this self-exaltation, he decided to lead a rebellion against God, and set up a hierarchy (Rev 12:3) similar to the one he will influence during the Tribulation, and take as many of God’s children away from Him as he could (Isa 14:12-14, Eze 28:14-17, Rev 12:3-4).
When this rebellion began, some of God’s children immediately took a stand for God and against Satan (Rom 8:26-39, 11:2-5, Eph 1:4-5). Then there were some who took a direct stand against God and for Satan (Rev 12:4). Aside from those two groups, we can deduce (because those who followed God He made His elect who cannot possibly stray from God, and those who immediately followed Satan were his most loyal who willingly became fallen angels who are sentenced to be destroyed, as they have chosen their destiny) that there was a third part who were lukewarm and could not decide who was worthy of their allegiance, but because of their indecision, were mostly deceived by Satan. This third group were essentially on the fence, and for the most part still are to this day until they care enough to decide to get off the fence and take a stand, either for God, or Satan.
As a result of this rebellion, God split eternity, eternity past from eternity future, and cleared off the earth and recreated it to insert the age we currently occupy, the Second Earth Age, and He mandated that each of us be born once in the flesh, through the womb of a mother (Heb 9:26-27). We would be born without memory of the world that was in the First Earth Age (2Pet 3:4-5), and this would be a test to each of us, in this controlled scenario, whether we would choose to follow God, or Satan by default (or even by choice) (Gen 1:2-31, Psa 104:5-14, Jer 4:22-27, 2Pet 3:5-9).
Because of the free will, which God gave each of us, there were some who chose not to be born through the womb, but instead continue to rebel against God’s will (Gen 6:1-4, Jud 1:4-6) and follow the same path that Satan took during this age. Of course, God also had His most loyal followers whom He chose from the time of the rebellion, and to these God would give a very important role while on earth in the flesh (Jer 1:5, Rom 8:28-30, 9:9-21, 11:2-5).
Our whole existence here on earth in these flesh bodies is a direct result of the Rebellion of Satan, and all of what we see and read in the Bible has a very specific part in God’s plan, which will ultimately end in an ideal Kingdom of Heaven led by God in His full glory after the Millennium.