Cardinal Emmanuel Suhard (Archbishop of Paris 1940-1949), from a retreat he gave to his own clergy, published in a book called Priests Among Men:
To be a witness does not consist in engaging in propaganda, nor even in stirring people up, but in being a living mystery. It means to live in such a way that one's life would not make sense if God did not exist.
Diogenes at CWNews has dropped that quote twice now. It's so good I need to post it so I don't forget it.