A famous, easily-used quote, from which Isaac Asimov obtained the title for his book The Gods Themselves. It comes from Friedrich Schiller's play Jungfrau von Orleans (The Maid of Orleans - ie, Joan of Arc).

The original German version is "Mit der Dummheit kämpfen die Götter selbst vergebens". Although it does require some training (unless your German is more fluent than mine), using this version instead of the English translation might spare you lots of trouble with your interlocutors (unless their German is just as fluent as yours). Use at your own risk.