John Milton's ten-thousand-line epic poem in blank verse, Paradise Lost, was first published in 1667 in an edition of ten books. The poem explores Adam and Eve's temptation by Satan, and their expulsion from the Garden of Eden for sinning against God.
In the poem, Milton portrays Adam as lonely, dissatisfied, and self-absorbed. Adam complains to God to provide him with a mate, so God creates Eve to satisfy Adam's yearnings for a companion.
Eve isn't exactly what Adam had in mind. She's self-possessed, strong-willed, and not at all submissive to Adam. Eve is more intelligent than Adam and much more curious about the world—Adam seems much more interested in Eve than in anything else—and it's Eve's desire for greater knowledge that leads her to be tempted by Satan to eat from the Tree of Knowledge.
Adam's blind devotion to Eve causes him to sin with her against God, and they're both banished forever from the Garden of Eden.
Milton presents God as omnipotent and omniscient, and as not having created Earth, Heaven, Hell, Adam and Eve, all the creatures of the Earth and all of the angels in Heaven out of nothing, but out of parts of Himself. In essence, God is in all things because all things contain part of God.
Milton's view of Satan is that of a tragic hero in the Aristotelian sense. Milton's Satan is a creature whose tragic flaw of pride causes him to rebel against what Satan perceives as the tyranny of God—the "Tyranny of Heav’n" (1.42)—and to proclaim that it's "Better to reign in Hell, than serve in Heaven" (1.263). This causes Satan's tragic downfall from God's favorite angel in Heaven to His most despised being in the depths of Hell. Satan's revenge against God is to bring sin into the Garden of Eden by tempting Adam and Eve to eat from the Tree of Knowledge.
Milton's portrayals of God, Satan, Adam, and Eve are in many ways antithetical to the prevailing teachings of the Catholic Church, towards which Milton was opposed.
It wasn't until 1732, however, during the papacy of Clement XII, that the Catholic Church placed an Italian translation of Paradise Lost on its Index Librorum Prohibitorum, the Church's list of banned books. The same entry banning Paradise Lost appears in all subsequent editions of the Index until 1900, by which time the ban against reading Paradise Lost had been violated on innumerable occasions.
The Index contains only a listing of banned books, but provides no explanation as to why a book is listed. Any such explanation remains secret in the Vatican archives.
The banning of Paradise Lost likely arises not only from the heretical content of the poem itself, but also from Milton's well-known anti-Catholic sentiments and anti-Catholic theology—including his questioning of the divinity of Jesus Christ and his rejection of the dogma of the Holy Trinity—which he expressed many times in his writings.
There is also the matter of Milton's criticism of the Index itself. In 1644 Milton wrote in Areopagitica; A speech of Mr. John Milton for the Liberty of Unlicenc'd Printing, to the Parlament of England that the Index was produced by "2 or 3 glutton friers [friars]" who "rake through the entrails of many an old good Author."
Interestingly, another of Milton's books, Literae Pseudo-Senatus Anglicani, Cromwellii, published in 1676, remained on the Index until the Index's final published edition in 1948, which listed over 4000 banned books. The Index was formally and finally abolished by Pope Paul VI in 1966.