Regarding season 1:
I found it to be very flawed from all angles and it only served as a learning experience for those who carried on with the second season. Its portrayal of a cult lacked a compelling thematic foundation, opting instead to draw inspiration from cheap rip-off D-grade movies themed around cults. Where cults are extremely over-dramatized to the point of crossing the supernatural territory. The cult apparently possessed super powers of hypnosis with only a few words spoken, even towards people who weren't perceptible to religious and spiritual beliefs. Everything surrounded the cult came off as cartoonish even within the context of a 'drama'. The narrative itself struggled to carry its own weight, particularly as it primarily focused on the perspectives of the outsiders. It always felt as if it was building up towards something bigger but it never got there. For a 16-episodes run, it needed a proper world building and foundation for the cult. Establishing a distinct "character" for the cult would have provided a stronger identity. Back to the outsiders, having the drama led by teenage characters was another weak aspect of it. Bland and cardboard characters. The mystery element was a crucial part of the drama yet it was lacking and under-delivering. The first season would have fared better as a standalone one-off movie rather than a drama. It doesn't make any sense why the drama was rated this high, according to MDL standards, especially when season 2 effectively addressed most of the issues with the cult and delivered an overall better package!

Regarding Season 2:
It managed to deliver a much more compelling package than season 1. The most important part is that it handled the cult theme properly. The characters and the cast were a lot more appealing than season 1. Having a more adult leading characters rather than teenager was a plus. Uhm Tae Goo was a perfect casting choice for his character. The story had a better and more natural progression than season 1. However, it got sloppier the closer it got to the ending. Speaking of the ending, it was a bit underwhelming and not as grandiose as I hoped it would be. Second season should have been a stand alone drama unrelated to the first one, not even in the name. If you don't feel like watching two seasons then you can ignore the first one and just watch the second.