The Bible doesn't say much about the rebellion of Satan and the other angels who followed him, but from the verses we do have it can be deduced that Lucifer, who was most likely the highest of the angelic host (Ezekiel 28:12-18), decided to try to take God's throne and become God himself (Isaiah 14:12-14). It is commonly understood from Revelation 12:4 that a third of the angels were in support of Lucifer and followed him when he fell.
Why did God allow the Satan and the demons to sin?
God offers no explanation as to why this occurred, but we do know from elsewhere in Scripture that God refers to the angels that did not fall as his "elect angels" (1 Timothy 5:21). Apparently, just as it is with humanity, the angels were found to have either of two invested natures, some with a nature that loved God and sought to follow Him, and some that when given the chance to rebel, took it. We do not know why God chose to do things this way. We can only assume that in His infinite wisdom, He deemed it to be the best way. The Bible makes it clear that God did not force the fallen angels to fall, for He cannot tempt anyone to evil and is not tempted to evil Himself (James 1:13), so it must be assumed that the demons fell because it was in their nature to do so, and when given the choice to rebel against God, they did what they desired to do.
Why then, did God in His omniscience create beings with the tendency to fall and rebel against Him in this way? Again, the Bible does not tell us God's reasons for doing what He did. But we do know that human beings choose to rebel against God unless He intervenes and creates a new heart in them (Ezekiel 36:26; John 3:3; Ephesians 2:8-9). Possibly, it works the same way with the angels; those not restrained by the gift of a godly nature will rebel when given the choice to do so.
It is puzzling, from a human perspective, when we consider that God foreknew these things and still allowed them. However, God's nature is exhibited in everything. His wrath and justice as well as His love and grace and mercy are shown through the existence of evil and rebellion in ways that never could occur without them. If, as the Psalmist says "His way is perfect" (Psalm 18:30), God has committed no wrong in allowing the angels to sin. In fact, this allowance is part of His perfect plan. Perhaps the best way to understand why God allowed the angels to sin is to remember that everything God allows will ultimately glorify Him in some way – even when a finite mind cannot understand how (Isaiah 55:8-9).
What are demons according to the Bible?
Who is Satan?
When was the fall of Satan, and how did it happen?
Why does God allow Satan and the demons to attack us?
What is going to happen in the end times?
Truth about Angels