God cast Satan out of heaven because Satan, through pride, lust, and greed, wanted to usurp God’s heavenly throne. Satan wanted to be as God at the very least, and also above God. He was full of pride because of his beauty. First we’ll consider the biblical references to Satan’s expulsion then some discourse in Paradise Lost (by John Milton) concerning this expulsion.

God created Satan as a beautiful and talented angelic being – a covering cherub, as the Holy Bible states. However, the Bible states:

 “Your heart became proud on account of your beauty, and you corrupted your wisdom because of your splendor. So I threw you to the earth; I made a spectacle of you before kings.”

            [Ezekiel 28:17 – New International Version (NIV)]

Satan was cast to the earth, as well as those angels (now demons) who chose to follow his corrupt ways. A third of the angels listened to Satan’s devious plans and followed Satan, hoping to seize God’s throne and rule the universe with Satan. The demons (formerly angels) fell to the earth with Satan – essentially evicted from heaven.

In Isaiah 14:13, it states that Satan desired to be the leader in heaven above all the angels.

“You said in your heart, "I will ascend to the heavens; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon.”

[Isaiah 14:13 - New International Version (NIV)]

In Paradise Lost, on page 114 of my hard copy version (The Works of John Milton – The Wordsworth Poetry Library) it states that the “infernal Serpent”, Lucifer who became Satan:

what time his pride

Had cast him out from Heaven, with all his host

Of rebel Angels, …

Therefore, we see here that Paradise Lost’s rendition of this event synchronizes with the biblical account. Furthermore, the account continues on this page 114 that Satan did indeed conspire to “set himself in glory above his peers.” Pride was certainly the downfall of the Devil and this is corroborated in the biblical account and in the epic poem Paradise Lost. On the next page in the Wordsworth edition of Paradise Lost, its states that God (the Almighty Power) cast Satan downward to earth so no profane and immoral being would inhabit the holy expanse of Heaven. It states that God expelled Satan and he was hurled:

… headlong flaming from the ethereal sky,

With hideous ruin and combustion, down

To bottomless perdition, there to dwell

In adamantine chains and penal fire,

Consequently, it is now obvious to the reader of the biblical account and the Paradise Lost account that God would not give space to rebellion in his Holy Heaven. Lucifer and his demons had freedom of choice, just as human beings do. He is suffering the consequences of the choice he made – his demons are as well. In Paradise Lost, Satan subsequently conspires to deceive Man and he also desires to regain his former exalted position in Heaven.

Satan wanted to be God. He thought that he was so excellent and wonderful that he should sit on the throne of Heaven. However, a created being is not, and cannot be, above the One who created him. Satan was cast out of Heaven because of his rebellion against God and God’s Government and Laws.

Why was Satan expelled from Heaven?

Satan was a cherub, one of God's most powerful angels, who openly rebels against God. Satan is jealous of God's omniscient status and pridefully wants to be exalted above God. Satan believes that he is God's equal and has the ambition to pursue his vain wishes. In a sense, Satan chooses to be expelled from heaven because he thinks "It is better to reign in Hell than to serve in Heaven." Satan's pride is responsible for his choice. He has faith in his own intellect above God's sovereignty and does not want to "serve" the Most High. Satan is content with attempting to ruin God's creation, humans, as a way to revenge his loss. In the book of Isaiah, Satan's intentions are described as it is written,

"For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: I will ascend above the heights of the clouds; I will be like the most High."(Isaiah 14.13-14)

The anaphora "I" at the beginning of each verse reveals Satan's prideful nature. He is exclusively concerned with how he feels and believes he should be exalted instead of serving God. After Satan falls from Heaven with his band of angels who choose to follow him, he accepts his fate and does not repent.