Wordsworth seems to be lamenting the state of London in 1802. He feels that her heroism of England's past has been "forfeited" and that the country has become morally and artistically stagnant, like a thick and dirty swamp. People no longer feel happiness they way they once did, and they require a leader who is righteous and heroic to lead them out of this era and into a new. The speaker says,
We are selfish men;
Oh! raise us up, return to us again;
And give us manners, virtue, freedom, power.
Thy soul was like a Star, and dwelt apart[...]. (lines 6–9)
He feels that Milton carried himself differently, that he had manners and virtue in a way that people seem to lack during his own lifetime. He longs for a return to those values rather than the ones that have led England to become such a "fen / Of stagnant waters." The theme, then, is that powerful leaders are required in difficult times, or even that, when society has been allowed to stagnate, it takes effective leaders to prompt progress.