The idea whose time has come.

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