The show started well: there was some action, reasoning behind the monsters' arrival and a presentation of the weird cult with a creepy leader... and then it went south. I think that it is sad that the potential of this drama was completely ruined by the lack of reasoning behind everyone's actions. Everything started to make less and less sense, and by the end of the show, it was extremely annoying to watch the characters make any decisions.
The "streams" of one of the church's fanatics were obnoxious and I think they did not add much to the story, except for the "plot twist" at the end, which was very predictable.
Overall I wouldn't recommend this show to any fan of Kdramas, but I may use it to warn someone about the possible outcomes of religious fanaticism,