Okay first I'll address the stuff seen in this kdrama:

-great friendship between the guys
-The female lead has really strong willpower
-a perverted old man (we talking around his 60s or70s) trying to pursue the heroine
-suicide/murder/bullying/sexual assault exist
-people blindly following a fake religion later a certain god "almighty" and goes psycho along the way
-other stuff I forgotten
-not meant for young audiences

Advice: If you're sensitive from what's listed, I would say save yourself from watching or if you're still willing to watch it then go for it (I wish you luck)

Beginning:
At first I didn't intend to watch this kdrama due to the poster (A closer look seeing an old man touching a young woman which creeped me really much but was intrigued to watch to see how the rescue goes and view how a cult works) Given that this is a thriller, the first scene fits for a suspenseful drama along with the atmosphere. Since I'm no professional with judging acting though a little familiar with OSTs (the primary subject that I usually focus on when watching and rating). It may be a little peaceful at first, but the action came in right away. Based off the first episode, I could tell that this drama wasn't for everyone but likewise anyone interested into dark topics you're more than welcome to watch it.

Around the Middle:
Well-paced and timing, ost gives off creepy and suspenseful vibe which is very nice. Really liked how one of the main male leads studies law since it became helpful with his willpower and confrontation with his confidence and the other which is action first, brain later undergoes the danger in order to help his friends rescue the heroine. Other than that, there were moments that had to be skipped aka when the perverted KFC man appears (he reminded of the KFC man for some reason) even when the cameras went closer to him I couldn't handle seeing him and what he did to the heroine and his purpose to having her be at the church 24/7. I'll say the process behind the rescue was the most interesting and what might drive people mad would be the civilians praying like crazy for this "almighty" god and doing harmful rituals and believing the living water is special water.....and where it came from is a bit disturbing or it was just an excuse to make them believe and enter the religion more which of course was bad or it was just plain drinking water.

End:
The downfall of everybody who operated the church-of course they met their ends except for one but thank god KFC went elsewhere and the heroine being safe and being friends with the boys afterwards was a satisfying ending though questionable at the final scene (after credits) It would be interesting if they included the reason to the old man's origin of becoming a psycho and a religious person so that it would explain why he created a cult and brought his buddies to help him unless it was due to his "love drive" and longing for young women to do "you know what" other than that I might have changed my rating for the story.

Overall it's an interesting drama showcasing a cult and how a victim undergoes being in one due to lost hopes from her parents. For me, it'll be a small chance of rewatching this kdrama since it wasn't the type of show I would watch, preferably read the dark contents than watch for various reasons (manhwa reader here). Final note: reminder this is just my opinion and have a nice day :)