How is Paradise Lost considered an epic? Provide three reasons.

An epic is a long poem with a heroic or elevated theme, usually involving warfare or conflict, that focuses on upper-class characters and often starts in media res, or the middle of the action. Homer's Iliad and Odyssey are the paradigmatic epic poems. What separates an epic from a merely heroic poem is its length and the rich elevation of its language. By elevation of language is meant using complex and allusive language and structure.

Milton follows the epic format in framing humankind's fall and redemption through Jesus Christ as a battle between heaven and hell. As in a classic epic, the gods (or this case, God) have already determined the fate of the players, but the interest comes in watching the details as the story unfolds. The adventure begins in media res, with a stunned Satan waking up in a lake of fire and brimstone after having been defeated in battle by God.

In Paradise Lost, two larger-than-life figures face off in a battle for the souls of the epitome of God's creation, humans. Satan, knowing he can't win against God in face-to-face battle, plots an underhanded way to defeat God's purpose and revenge himself for his defeat: he will corrupt humankind. This battle is played out in typical epic form by retelling a familiar story (in this case, of the biblical Fall in Genesis) in immensely rich, Baroque, elevated language. As is typical of an epic, too, the suspense in Paradise Lost builds, although we already know the outcome of the story.

God and Satan are both depicted as monarchs, though God is a true and perfect king, and Satan is merely a debased parody of kingship. Satan, however, reaches heights of grandeur perhaps more than Milton expected, causing the Romantic poets, in particular, to see him as the hero of the piece.

Why is Paradise Lost considered a literary epic?

An epic is a long story narrating the adventures of a heroic figure. Oftentimes, this hero is related to the overall fate of a nation or culture, such as in The Epic of Gilgamesh, The Iliad and The Odyssey, Sundiata, War and Peace, and The Dream of the Red Chamber. Traditionally, epics were poems or from oral traditions, though in recent centuries, novels and films have also had their share of epics.

Paradise Lost is considered a literary epic because it fits just about all of the classical, qualifying criteria. It is a long poem written in an elevated style meant to give weight to the events depicted. It details the adventures of heroes and villains. It focuses on grand themes: good and evil, sin and redemption, love and lust, and justice and mercy. While it does not tell a story linked to any particular nation, it is linked to a religious culture: specifically, Christian culture since it tells the story of the fall of mankind in the Book of Genesis.

In many ways, Paradise Lost could be considered Milton's answer to the epic poetry of the classical world. He takes hefty inspiration from The Iliad, The Odyssey, and The Aeneid while adding his own Christian theology into the mix. Unlike the pagan heroes, who are heroic for their physical strength, cleverness, and drive to make a name for themselves, Adam and Eve (arguably the true heroes of Paradise Lost) are ordinary human beings who must rely on God's strength and not their own. This is a fully Christian notion of heroism. Ironically, Satan more closely resembles the classic epic hero of antiquity, but his strength, brains, and hunger for glory are corrupting rather than ennobling.

Why is Paradise Lost considered a classic?

One thing that defines a piece of literature as a classic is its influence on other writers. Paradise Lost has had that kind of lasting influence on such writers as William Blake, Mary Shelley, and C. S. Lewis. One critic has noted that

More than any other literary forebear, John Milton's Paradise Lost stands as a continuing intellectual and mythic reference point for Mary Shelley's Frankenstein. (see the Univ. of Penn. link below)

To many people during the 18th and 19th centuries, Paradise Lost was almost as important to have as the Bible. The Norton Anthology of English Literature states that "Paradise Lost was often one of the very few books found in British and American households."

See also the eNotes critical overview of the poem (linked below).

Aside from Beowulf, Paradise Lost is the first poem of epic proportions to be published in England. 

It is considered a classic for several reasons, one of them being the enormity of the task in rewriting the creation story of the Bible.  The language is beautiful and captivating, and he tells the story from different points of view--Adam, Eve, the Serpent, etc.

In addition, as with any "classic", Milton asks the important and difficult-to-answer questions that others may shy away from due to their controversial nature.  Some of these are listed in the enotes summary, and are as follows:

If the world was created by a good, just, and loving God, why is there little evidence of goodness and justice in the world? What does it mean for humankind to be created in the image of that God, and how does humanity endure in a fallen world?

Follow the link below to read more about Paradise Lost.