Cult of those who have gone out without Navigation and gone mad/been twisted physically, 'The Wayward'? Follow the tenets of the cult leader, a single man, 'The Lost Child'? or should it be 'The Lost Children/Disciples' lead by 'The Wayward Man' or just 'The Wayward' or something similar?

In any case they're the main threat (along with terrestrial beasties who've gone a bit strange in the wilds) when Navigating. They think being 'lost' is a good thing and think the cities should come down so everyone can be 'Lost' and live among the wilds again. 

This is because the main leader is actually the one who CAUSED the cataclysm? He was mapping out beyond Earth and realized there are alien races/cosmic creatures/celestial beings waiting for Humanity to leave Earth/become space-faring in order to prey on them/enslave them/etc. 'The Monsters In The Dark'? the 'dark' being space. something that the cult talk about a lot, but is dismissed either as insane rambling, or mistaken for being self-referential?

By causing the cataclysm humanity would be cut off from itself but also the things waiting beyond Earth, and therefore safe. 
Finding the Atlas will not only re-map the Earth for Humanity, but also map the stars and the solar system and lead Humanity on an inexorable journey beyond the planet - and towards the things waiting there for them.