What is the main theme of Paradise Lost?

One of the main themes is that of sin. It is important that, while God made Adam and Eve "sufficient to have stood," he also created them "free to fall." This freedom is rooted in then-very nature, for God "formed them free, and free they must remain" (III. 124). For Milton, humankind's obedience is proof of their love and service to God, and obedience therefore must be free, for obedience which is not free is not obedience but slavery. It is for this reason that Adam is free to fall, and for this reason that he must leave Eve free to make her own choices, even if that choice be to leave him and work alone and vulnerable, "for thy stay, not free, absents thee more" (IX.372). But in this very freedom lies the possibility of disobedience, for in the freedom of will lies the possibility of choice.