And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels, And prevailed not; neither was their place found any more in heaven. - Revelation 12:7-8 KJV
Defiance against divine sovereignty together with the selfish desire of being equal to God, triggered the conflict of the ages between Christ and Lucifer.
The rebel angel lost the battle and was cast out of heaven:
And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. - Revelation 12:9 KJV
Jesus told his disciples:
And he said unto them, I beheld Satan as lightning fall from heaven. - Luke 10:18 KJV
Based on I WILL GO by Alejandro Bullón