Paradise Lost, mirroring Homeric epics like The Aeneid, begins in medias res, that is, in the middle of things--after God has already cast Satan and his followers out of heaven and into a pit of fire. Milton conceived of Paradise Lost as a contemporary epic poem about man's fall from grace and the struggle between good and evil, personified by God and Satan. Important elements of Milton's epic include heroic language and settings, catalogs of mythical beings, the names of those engaged in the essential struggle, life and death consequences of victory and defeat, and epic similes.
Appropriately, the first epic simile in Book I describes the poem's main character, Satan:
[Satan] lay floating many a rood, in bulk as huge/As whom the Fables name of monstrous size,/Titanian, or Earth-born, that warr'd on Jove,/Briareos or Typhoon, whom the Den/By ancient Tarsus held. . . .(ll.196-200)
Milton establishes, again using an epic convention, that Satan is as large as one of the Titans, the physically huge precursors of the Greek and Roman gods who actually went to war with their children, gods such as Zeus (also, Jove), over control of earth and mankind. Milton's readers would understand the war between Zeus and his fellow gods and the Titans as analogous to the war between Satan and God.
As Satan moves out of the lake of fire, his shield, hung on his shoulders, is described in epic terms: "the broad circumference/Hung on his shoulders like the Moon, whose Orb/Through Optic Glass. . . ." This simile is particularly important because it emphasizes the large scale of everybody and everything in the poem: just as Satan himself is so large that he covers "many a rood" (a rood is equal to about 7 yards), his shield must also be equally large to protect his body and is therefore compared to the moon.
One of the important similes in Book I establishes the magnitude of Satan's fallen angels:
As when the potent Rod/Of Amram's Son [Moses]. . ./up call'd a pitchy cloud/Of Locusts, warping on the Eastern Wind. . ./So numberless were those bad Angels seen/Hovering on wing under the Cope of Hell (ll.339-345)
Here, Milton establishes not only the size (numberless) of Satan's army of fallen angels but also compares them, in terms every reader would recognize as negative, to the locusts called up by Moses in order to get the Pharoah to release the Israelites from bondage in Egypt. Any reader of Paradise Lost tempted to sympathize with Satan and the fallen angels is reminded that they are no different from the plagues in Egypt. In the following passage, another simile compares the fallen angels to the barbarian hordes who came from northern Europe to terrorize the cultivated societies of southern Europe, another comparison that would resonate negatively with an audience in the mid-seventeenth century.
These are only a few of the epic similes Milton employs to set the stage for the conflict that follows in Books II through XII, and although at some points, some similes describe Satan as truly heroic, Milton's goal in using such similes is to create a Christian epic, using the conventions of the Homeric epic.