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?
This is actually from the book Alamut that is about a fictionalized version of the historical Assassins. Since the book was published in the 1930's, there is some possibility that Brothers Karamozov was an influence, though there is an interesting inversion that instead of the moral relativism of a world without God, it is the personal ethos of a sociopath that is using a radical religion to gain unquestioning loyalty and obedience of a group of men for the sake of power.