“There is nothing more corrosive to a democracy than the idea that there is no truth.”

Open Letter to the Committee Hearing Re: FBI Director James Comey and National Security Agency Director Mike Rogers

Ron English's Fauxlosophy (2016)

Propos sur le Religion no. 74 (1938), under the pen name Alain.
Alternate translation: “Nothing is more dangerous than an idea, when it's the only one we have.” IZQuotes https://izquotes.com/quote/%C3%A9mile-chartier/nothing-is-more-dangerous-than-an-idea-when-you-have-only-one-idea-390165 (retrieved 10/30/18).

Maxim 715, trans. Stopp
Maxims and Reflections (1833)

Source: The Darkness That Comes Before

Often attributed to Hugo as a paraphrase of a similar idea in his Histore d'un Crime (1877): "One resists the invasion of armies; one does not resist the invasion of ideas", the wording of this famous statement actually more closely resembles a passage from the relatively obscure Les Francs-Tireurs (1861) by Gustave Aimard, p. 68 https://books.google.com/books/about/Les_francs_tireurs.html?id=mKI4AQAAIAAJ:
Il y a quelque chose de plus puissant que la force brutale des baïonnettes: c'est l'idée dont le temps est venu et l'heure est sonnée.
There is something more powerful than the brute force of bayonets: it is the idea whose time has come and hour struck.
Translated into English as The Freebooters : A Story of the Texan War (1861) https://archive.org/details/freebootersstory00aima, p. 57, Ward & Lock edition
Misattributed
Variant: More powerful than the mighty armies is an idea whose time has come.

Late Night with David Letterman (5 March 1993).

My Heart's in the Highlands (1939)