This drama was a mess, at the beginning was difficult to understand and then when you can start guessing what is happening, everything is just bad, messy and without following the "original" plot line (to say everything changes from episode 4 onwards and the story just gets more uninteresting), there's a lot of plot holes, honestly, the visual effects are horrendous (no comments), there's some gore (not really, to be honest, I have seen more gore than this), no suspense or thriller.
The character's backgrounds, don't expect to be explained (they aren't), characters appearing, disappearing with no apparent explanation (there's little reasoning for this drama), some characters are underdeveloped that could have had more potential, others that could have been better without them...
The ending, it's difficult to explain, you could take it as a cliffhanger or not, because precisely the main character's story/problem is solved in the end so there isn't exactly a need for a season 2, the cult will be disbanded as it has lost it's followers and the problem with the monsters aka demons won't be solved, how can humans solve supernatural problems, even more so being that problem a hell one?
So what with that ending scene with the reborn woman? That it is, they are reborn if they are supposed to, I don't Chairman Jung will be reborn (as he is precisely the demon personified).

That's all for me thoughts in this hellish drama.
I don't recommend it, don't waste time on this one, even if the mc's are your favourite actors, it's not worth it, believe me I fell for that trap.