Percy Bysshe Shelley

From Memory Alpha, the free Star Trek reference.

Percy Bysshe Shelley was an Earth poet. He wrote, "All spirits are enslaved that serve things evil".

Captain Jean-Luc Picard quoted this line in a conversation with Armus. Picard's point was that to do the bidding of Armus would enslave the crew of the USS Enterprise-D. Armus took the quote literally and argued that he wasn't a servant of evil, he was evil itself. (TNG: "Skin of Evil")

The poet is not referred to by name in the episode other than as a great poet. Picard is quoting from the poem Prometheus Bound, line 110. A fuller article of the poet can be read at Percy Bysshe Shelley.