If we read the first few lines of Paradise Lost, we can see the poet mentioning the theme of the epic quite clearly. The poet says, "of man's fist disobedience…", which means that the subject that he is going to address is man's first disobedience; the disobedience "which brought death into the world". It is very much evident from this point onwards that the real point of debate is the action of Adam and Eve due to which Man had the mightiest fall. This fall resulted in the loss of the Eden Gardens. Then came "a greater man…and restored that blissful seat".
Another aspect of Milton's Paradise Lost is that his ambition was to "justify God's ways to man". But many of the critics say that in doing so, Milton unconsciously moved towards making Satan the hero of Paradise Lost. This comprehension of the epic takes back the theme from the poem, on which Milton started to write.
On the other hand, the progression of the epic, the forward and backward movements of the epic also justify the theme of justifying God's ways to man. In Book I, we can see Satan and his marvelous speeches only. But as we move down in the epic, we see that in Book IX, all the heroic stature of Satan has been taken back and he is confined to a level of a Serpent only. So, by diminishing the character of Satan, Milton wants us to visualize the real essence of the epic.
Isn't freedom involved in the theme, as well?