In the Garden of Eden, Adam and Eve had everything they could ever want. They lived in a true paradise in which, thanks to God, they wanted for nothing. Life was calm, blissful, and entirely free of trouble. And if humanity's ancestors—according to Genesis and Milton, at any rate—had behaved themselves and continued to obey God, then it would've stayed that way.

But of course, it didn't. Adam and Eve foolishly disobeyed their creator. And in doing so they lost paradise, hence the title of Milton's epic. They ignored his strict command not to eat the forbidden fruit of the Tree of Knowledge. By doing this, they brought sin and death into the world. As paradise had now been defiled by their selfish actions, there was no longer any place for them in the Garden of Eden.

As a result, Adam and Eve were forced to leave paradise, with disastrous consequences for future generations. From now on, humankind would be burdened with the taint of original sin, ensuring that no human being would be able to live in an earthly paradise ever again. By defying God, Adam and Eve lost paradise, not just for themselves, but for everyone who lived after them.