The light of a lamp shines on and does not lose its radiance until it is extinguished. Will then the truth, justice, and self control which fuel you fail before your own end?
The "gods or atoms" argument is displayed again in chapter 14 of Book 12. The gist of this idea is that the action is the same, whether you believe in a god or gods or if you do not. The end result, "govern yourself." If you can govern yourself, in the maelstrom, then do it. If things are so random and chaotic, fine - accept it. But then proceed to organize your mind.
Like light, the virtues of truth and justice and temperance always exist. But do they exist in you? You need to light them within you and they will always burn and light your life as long as you don't extinguish them. And the choice is entirely yours as to whether you extinguish them or let them burn on.
(see also Citadel p. 45, 113, 148, 156-157, 237)