Milton is often villainized for his misogyny, his misanthropy, his solipsism. While he may have tended toward disagreeability in life—he was certainly a cranky old man—even a cursory reading of Paradise Lost reveals a man of deeply humane sentiments. His treatment of Eve prefigures many modern ideas about the role of women in society, and his understanding that Adam acted as a free agent rather than a dupe of Eve controverted many contemporary theological notions. But Milton's epic poem isn't just about people; in twelve books he chronicles the war in Heaven, the fall of Satan, creation, the Fall, and Christ's ultimate redemption of all things.
The poems that survive to succeeding generations most often display both adherence to traditional forms and innovation. In his poem, Milton took the blank verse commonly used in drama and wrought one of the greatest, most original literary masterworks in the history of the world. Its scope alone is more than most writers could handle, and the fact that he manages it with such grace and such an elegant yet restrained style (the more impressive given the verbosity of his contemporaries) ensures its lasting importance. Beyond that, Paradise Lost has inspired hundreds of years of novelists, poets, thinkers and theologians—which is ultimately the true test of a work's greatness.
No one element of this poem is better than the others, but by far the most intriguing character is Satan, famously portrayed here as a tragic anti-heroic figure of the classical variety, brought low by his own hubris. Later Christian tradition has made of Satan a horrible monster, but Milton's version reveals the satanic element in man's own nature, and extends the metaphor to encompass all forms of rebellion: man against God primarily, but also demonic rebellion, the revolt of nature, and even man's own conflict with himself.
What's surprising to many modern readers is how fresh Milton's style remains. Paradise Lost isn't filled with archaic language and obscure references, it maintains its narrative pace easily and takes the reader from heaven to hell to earth and back again, with never a weak line, never a phrase that seems out of place or contrived. Part of this may be his apparent disdain for pretension; he never embellishes more than is necessary, never becomes drunk with his own brilliance. Throughout the focus is Christ as the One come to restore the paradise of Eden through His redemption of all things. Even in this Milton remains moderate, refraining from preaching or too many overt references.
Some of the greatest works in the Western literary canon are those exploring the deepest themes of Christianity—The Divine Comedy, the poems of John Donne, Chaucer's Canterbury Tales—and Paradise Lost is certainly among their number on both counts. Nowhere else is man's genuine glory and defeat better portrayed, nor his ultimate redemption and freedom. That Milton wrote the entire poem while blind, dictating from memory passages already devised to casual visitors, only affirms these themes and increases our admiration for a great thinker and Christian who was also one of the finest artists of any era.