Theme: "To justify the ways of God to man.'' This doesn't mean bringing God to book, but revealing the divine purpose. Milton saw his dream of a Puritan Commonwealth perish with the Restoration of Charles II - original sin guarantees that humanity will seek enslavement.
Style: Usually described as Latinate, it's actually idiosyncratic. The syntactic inversions are often for an ironic purpose, the grand vocabulary an attempt to compose an epic music.
What they thought of it then: Magnificent but old-fashioned - which it would be, if your taste also ran to erotic cavalier poems larded with smut. Dr Johnson said he wouldn't have wanted it a line longer.
What we think of it now: TS Eliot and FR Leavis complained that Milton lacked both an eye and ear for poetry. But they were both keen on a poetry rooted in the vernacular. More sensible to assume that Milton is second only to Shakespeare.
Chief strengths: Out of improbable raw material, Milton created a poem of encyclopedic range and intense dramatic energy. It is not only epic but lyric, pastoral, satiric and tragic by turns. An introduction to a way of thinking both aggressively challenging and refreshingly alien.
Responsible for: the Romantics' love of Satanic anti-heroes. And the dark millionaires who haunt the pages of Mills & Boon.