Pride, the first sin.
He was the most beautiful angel. He wanted to be a ruler himself, usurp the throne of God.
He deceived 1/3 of the angels to join in his rebellion against the LORD.
They were cast out and Lucifer became known as Satan. The fallen angels turned into demons and evil spirits.
Satan then deceived mankind and now he is the "god" of this world, the power of the air (UFOs/ghosts/etc).
Occult societies such as the Illuminati and Freemasons control the world and do the bidding of the devil.
Satan continues his rebellion against God by corrupting mankind spiritually, physically and mentally.
Pre-flood genetic tampering, Tower of Babel, Mystery Babylon, false religions, etc. Satan is the "god" of this world, the vast majority of people are hell-bound.
As Christians, we are pilgrims in this world. We live in it, we're part of it, but we're not of it. Our home is an eternal and glorious one with God Almighty and our Lord Jesus Christ.
Satan has already lost, his plans for a New World Order is futile.
Basically, as Christians we are soldiers behind enemy lines. We have to resist and stay strong in the faith until Christ's 2nd Advent.