The Exordium[lines1-26] of Milton's famous epic Paradise Lost states the subject/theme of the whole poem:'Man's first disobedience' and the consequent fall of man from heaven--the Biblical story of 'Adam Unparadised'.
But Book I of Milton's poem deals with the story of another fall: the fall of Lucifer/Satan and his followers[rebel angels] from heaven into the bottomless pit of hell to suffer endless perdition.The brightest of angels, proud & ambitious, disobeyed the Almighty and waged 'an impious war' in heaven. He suffered from 'a sense of injured merit' and misled a large number of angels to a regicide. Satan and all his followers were struck with thunderbolt, and they were 'hurled headlong flaming' into the dark & fiery pit of hell. The narrative begins with their fall, with Satan regaining his consciousness to discover himself confined in a 'dungeon horrible'. As Satan further discovers his second-in-command, Beelzebub, still lying unconscious close to him, he addresses Beelzebub to begin the process of remobilising his forces for a new offensive. Book I contains five speeches of Satan addressed to Beelzebub and the other followers lying defeated and scattered all over the burning pool in hell. We have Beelzebub's reply followed by Satan's self-disentanglement from the waves of the fiery gulf and flight to the solid land ashore. Then the fallen angels return to consciousness, re-assemble, and march in battle order in response to the call of their leader. Then Satan's architectural brigade builds 'Pandemonium' in hell where the new king of hell sits with his compatriots in 'a secret conclave' to work out the new mischief: to tempt the loved creatures of God--Adam & Eve--to disobedience and fall.
The beginning of Paradise Lost is similar in gravity and seriousness to the book from which Milton takes much of his story--the book of Genesis, the first book of the Bible.He opens Paradise Lost by formally declaring his poem's subject, human being's disobedience towards God and the consequences that followed.The first words of Paradise Lost state that the poem's main theme will be “Man's first Disobedience.” Milton narrates the story of Adam and Eve's disobedience, explains how and why it happens, and places the story within the larger context of Satan's rebellion and Jesus' resurrection. Raphael tells Adam about Satan's disobedience in an effort to give him a firm grasp of the threat that Satan and humankind's disobedience poses. In essence, Paradise Lost presents two moral paths that one can take after disobedience: the downward spiral of increasing sin and degradation, represented by Satan, and the road to redemption, represented by Adam and Eve.
While Adam and Eve are the first humans to disobey God, Satan is the first of all God's creation to disobey. His decision to rebel comes only from himself—he was not persuaded or provoked by others. Also, his decision to continue to disobey God after his fall into Hell ensures that God will not forgive him. Adam and Eve, on the other hand, decide to repent for their sins and seek forgiveness. Unlike Satan, Adam and Eve understand that their disobedience to God does not know that their disobedience will be corrected through generations of toil on Earth. This path is obviously the correct one to take: the visions in Books XI and XII demonstrate that obedience to God, even after repeated falls, can lead to humankind's salvation.