"The greatest trick the Devil ever...

"The greatest trick the Devil ever pulled was convincing the world he didn't exist."

Note that the quote isn't entirely original:

1836: John Wilkinson, Quakerism Examined.

One of the artifices of Satan is, to induce men to believe that he does not exist.

1864: Charles Baudelaire, "Le Joueur Généreux" (“The Generous Gambler”)

La plus belle des ruses du Diable est de vous persuader qu’il n’existe pas!

Translated:

The greatest trick of the Devil is to persuade you that he does not exist!