Its more than a simple belief that there is good and that it should fight the evil in the world. It's a personification of Light and Darkness at their most elemental level, as forces that are so absorbed with themselves that one cannot exist without the other though they constantly try to consume one another. One of the earliest repersentations of Light and Darkness was of Light being a massive black bull and Darkness being an enormous white bull.
P.C. Cast, Burned
wisdom