Kinda cringe for an origin of a creed, but it's a good rallying phrase. Surprisingly succinct.
"To say that nothing is true, is to realize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted, is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic."
Are there any similar phrases you've seen popularized by a game?
If you generalize it a tiny bit, you get "Nothing is absolute; everything is possible". And that's basically the foundation of a constructivist philosophy.