Why have Satan and his followers been cast out of heaven in Paradise Lost?
In Books 1-4 of Paradise Lost, Satan is cast out of Heaven because of pride and hate. In a way, though, Satan actually chooses to be cast out. Choice is very important in Milton's ideology. So says Enotes:
God tells his Son of Satan's diabolical plan to seduce humankind, foretelling Satan's success and simultaneously clearing himself of blame. He contends that humankind was created free and able to withstand temptation, yet outlines his purpose of allowing humankind grace, since they will fall, not out of malice, as Satan did, but deceived.
In this way, Satan willingly converts from being an angel of light to a devil in complete darkness. After all, Satan believes:
"It is better to reign in Hell than to serve in Heaven."
Satan appears to be heroic, like a tragic hero: he blames himself for his fall; he is aware of his emptiness and fallen state; he prides himself on the ability to choose this fate.
Ironically, Satan is the most developed character in Milton's epic. He is the model Tyrant who fills his followers (Moloch, Belial) with hope, albeit false hope. Satan knows he will lose in the end, but he nonetheless summons the courage to fight. In this way, Satan becomes heroic. (Later, the Romantics, namely Byron, will be fascinated with this concept of the "Satanic Hero"--a kind of "Sympathy for the Devil").
There is much speculation about who Satan is and why he was cast out of heaven. Answers vary from very simple to extremely complex, but the most important aspect of the entire issue has to do with sin. Most of the answers have to do with pride and wanting to be equal to God. He was an angel who grew jealous.
Satan or Lucifer created sin. God is incapable of being around sin because he is complete perfection. Thus, Satan had to go. It is really difficult to grasp concepts of religion truthfully and accurately because so much is based on faith. Fortunately, major religions have documents that have stood the test of time to prove their beliefs. The bible is your best resource for the answers to your questions.
The angelfire.com site cites the bible in many places. I recommend you click on the link, check out some of the references and read the context around them for a full understanding.