On résiste à l’invasion des armées; on ne résiste pas à l’invasion des idées.
One translation: No one can stop an idea whose time has come.
Another translation: While it is possible to resist military might, an invasion of ideas cannot be stopped.
– Victor Hugo, History of a Crime
http://www.quotecounterquote.com/2011/02/nothing-is-more-powerful-than-idea.html
http://en.wikiquote.org/wiki/Victor_Hugo