The greatness of Paradise Lost lies in Milton's elevated and grand style. Illustrate from Book One.

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Milton had soaring ambitions for Paradise Lost. He hoped to create an epic that would stand alongside the works of Homer, Vergil, and Dante, and that would be unparalleled in the English language. The theme he chose is a lofty one, and the language that he used is tailored to the subject matter, as he demonstrates in the beginning of Book One:

Sing Heav'nly Muse, that on the secret top
Of Oreb, or of Sinai, didst inspire
That Shepherd, who first taught the chosen Seed,
In the Beginning how the Heav'ns and Earth
Rose out of Chaos...I thence
Invoke thy aid to my adventurous Song;
That with no middle flight intends to soar
Above th' Aonian Mount while it pursues 
Things unattempted yet in Prose or Rhime.

This excerpt is a deliberate nod to Homer, who also included evocations of Muses. It also expresses his ambition to write of "Things unattempted," i.e., to "justifye the wayes of God to men."  He continues to employ grand, sweeping language as he describes the titanic struggle between God and Lucifer, one which left the rebellious angel cast down 

With hideous ruine and combustion down
To bottomless perdition, there to dwell
In Adamantine Chains and penal Fire,
Who durst defie th' Omnipotent to Arms.

It is, perhaps, precisely this majestic language that makes Paradise Lost such a challenge to modern readers, but it is also suited to the epic themes of combat between God and Lucifer, one in which man is swept up. It is fundamentally a struggle between good and evil, and Milton wants to show both the majesty of the former and the power of the latter. His success in doing so, and in reimagining a foundational story in Christian tradition in epic terms, owes much to his elevated style.