Charles Baudelaire.
The quote “The greatest trick the Devil ever pulled was convincing the world he didn’t exist” is widely attributed to Charles Baudelaire, though search results note it is a paraphrase of a phrase from one of his stories rather than a direct, verified line from the poet himself. This specific wording gained massive popularity after being featured in the 1995 film The Usual Suspects, where the character Keyser Söze (played by Kevin Spacey) delivers it.