Great build-up, then a pretty bad flop
With the first half of this book, I was almost 100% certain this would be a 5 star review from me. You get just the right amount of explanation for the cult inbetween what feels like just the right amount of character development to make you care about the protagonists. Not to mention a taste of the amazing suspense Nevill can dish out, wherein you're on the edge of your seat hoping the character isn't discovered. Then when everything starts going downhill for them and looking absolutely bleak, you get what feels like is going to be an amazing retribution for the characters.
Then the second half happens. After that previously mentioned taste of retribution, one protagonist is reduced to hiding with basically no suspense added onto this hiding. It's like she totally forgot any bit of fervor she previously had for the reprisal she deserved. The other protagonist seems like she attempts to continue that retribution, but you're left with a low-effort description after-the-fact of her getting picked up by the cops before it could even be offered. Just a bit before that, you get what comes off like an attempted, but very dissatisfying twist. This twist leaves things open-ended and in a VERY sloppy way, like Nevill decided he might write a sequel last minute in the writing process.
Finally, I could not believe how drab and uninteresting he made the cult's characters when he started devoting chapters to explaining the background of it from their viewpoints. The way he did it was so excessive, and thus killed off any horror and suspense the cult previously offered for the sake of the over-explaining. It felt like I left one novel for a short while and got transported into another, really dim one that I wasn't interested in reading.
tl;dr if you're new to Nevill's novels (heh), start with The Ritual, it's basically a much much better The Reddening, then maybe give the latter a try.