John Milton begins Paradise Lost with an invocation of the Muse, in true epic fashion. His exhortation of the "Heav'nly Muse" includes a lot of detailed praise and summaries of what he'll be talking about, and then he ends his proem with this:
What in me is dark
Illumine, what is low raise and support;
That to the heighth of this great argument
I may assert eternal providence,
And justify the ways of God to men.
Unlike Homer or Virgil, Milton expects to be changed by the Muse. As he stands, he's not up for the task of telling this story. It all goes back to the Miltonic project of building a person who can appreciate Scripture, which he sees as the height of truth and artistry. We have to be changed before we can love the Bible adequately, and Milton has to be changed before he can begin the process of changing us.
Milton wrestled with a syncretic problem — how to reconcile a love of pagan myth with a love of Christian truth — and more generally, a problem of why arts are useful at all. If the goal of life is to serve God so you can go to Heaven, why pursue artistic excellence? Milton's answer is that the arts, by training the imagination, can help us desire virtue, which we can then find in Scripture. He wants to create of his readers men of "true virtue," who can really appreciate what the Bible has to offer. Lest that sound like a simplistic, God's-Not-Dead kind of task, I should point out that the way Milton decided to do this was by making the devil the most interesting and appealing character in his epic.
Speaking of the devil, sometimes I run into people making arguments that Satan is the hero of Paradise Lost. I suppose you could make a case for that, but if you take a traditional approach, the same approach that gives you Achilles as the hero of the Iliad, it's pretty clear that Milton had a different hero in mind.
In the Iliad, the subject is the rage of Achilles. That's what kicks the action into high gear, and that's what brings the story to a close. In the Odyssey, it's Odysseus and his "poly-metis," his many-mindedness. He's too crafty for his own good, but the craftiness is what brings him home at last.
In Paradise Lost, the poet tells the Muse to sing of "man's first disobedience," so it would seem that the subject of the poem is the fall. But the proem ends with Milton's desire to "assert eternal providence, / and justify the ways of God to men." In true Boethian style, then, Milton's chosen subject is how Providence can be said to act in all things, including the most horrific event of all, "man's first disobedience," when sin entered the world. Essentially, Milton's subject is the problem of evil, how God can bring good out of bad, which is why, with the Muse's help, he wants to "justify the ways of God to men."
If that's true, who does that make the hero of Paradise Lost? The same man who's the hero of Paradise Regained — the only true man of virtue.