It's very close to Taoism or Buddhism - everything is an illusion, and you can do whatever you want. Beyond its potential for interactive gaming, the creed itself also served to pinpoint the forces that the assassins would find themselves aligned against. In the organized religion of the time, regardless of faith, the people were encouraged to believe that everything they were told was true, and almost nothing they wanted was permitted. In the mythology of the game, this is the idea that the assassins struggle against.