I'm reading Horus Rising (the first book in the Horus Heresy series) and the Luna Wolves are on their great expedition to reunite the galaxy and quash the primitive beliefs of the worlds they're taking over.

There are a few sections where they describe how fearlessly atheist they are.

Are they being naive or are they forgetting the chaos gods? It's my understanding that the four brothers were responsible for sabotaging the primarch project so the imperium must know of their existence?