“There is nothing more powerful than an idea whose time has come.”
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.
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Victor Hugo 308
French poet, novelist, and dramatist 1802–1885Related quotes

“No army can withstand the strength of an idea whose time has come.”
Variant: No force on earth can stop an idea whose time has come

“An invasion of armies can be resisted, but not an idea whose time has come.”

Paraphrasing Victor Hugo when speaking about the Civil Rights Act of 1964 http://www.senate.gov/artandhistory/history/minute/Civil_Rights_Filibuster_Ended.htm (10 June 1964)
1960s

“Ideas are more powerful than guns.”
Wikinews interview with Tony Benn (8 August 2007), quote from approx 24min45 sec into interview.
2000s

“There’s nothing more dangerous than a good idea.”
Ron English's Fauxlosophy (2016)

Context: Circumstances have been such, that I have lived almost entirely secluded for some time. Those who are much in earnest and with single minds devoted to any great object in life, must find this occasionally inevitable.... You will wonder at having heard nothing from me; but you have experience and candour enough to perceive and know that God has not given to us (in this state of existence) more than very limited powers of expression of one's ideas and feelings... I shall be very desirous of again seeing you. You know what that means from me, and that it is no form, but the simple expression and result of the respect and attraction I feel for a mind that ventures to read direct in God's own book, and not merely thro' man's translation of that same vast and mighty work.
In a letter to Andrew Crosse, as quoted in Eugen Kölbing's Englische Studien, Volume 19 https://archive.org/stream/englischestudien19leipuoft#page/157/mode/1up (1894), Leipzig; O.R. Reisland, "Byron's Daughter", p. 157.

“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

“Nothing is more dangerous than an idea, when you have only one idea.”
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).