Paradise Lost is full of allusions. Milton had a reputation for being extremely well-read, and he wove allusions, both classical and biblical, throughout his epic poem. For example, on the classical side, he often alluded to Ovid's Metamorphoses, as he does in Book IV when Eve first comes to life. Like Narcissus in the Metamorphoses she is attracted to her own reflection in the water, foreshadowing how her vanity will be her—and Adam's—undoing.

Of course, the biblical book of Genesis, the story of creation, is a foundational source for Paradise Lost, and while Milton interprets it in his own way, he also leans into its wording heavily. For example, in Book III, line 12, Milton describes the earth as formed from the "void and formless Infinite," which echoes the wording in Genesis 1:25: "the earth was without form and void."

Milton goes well beyond Genesis in his biblical allusions, as he goes far beyond Ovid in his classical allusions. The poem is so densely allusive that often annotated versions have more notes than text on every page.

There are tons of Biblical allusions, since it is basically the re-telling of the Adam and Eve story in the Bible and their fall from grace.

There are also Classical allusions to Greek and Roman mythology.

Check out the eNotes summary and critical analysis of Paradise Lost for more information, or just search "allusions and Paradise Lost" on Google.