I struggled between rating this 4 or 5 stars, but ultimately it came to how long it took to merge the 2 sides of the story. I felt like it took longer than expected, but that might be the only critique I have.
Everything else about this book, I loved! Being that it's about an ancient war among angels, it is very heavily Biblically based, even going so far as have Bible verses at the beginning of some chapters. I loved that! As a Christian, it was very cool to get major Bible events from a different perspective. However, if you're not religious, I think you would still enjoy the book. Since the story goes between the perspective of angels and humans, I don't think the Biblical part gets to heavy or overwhelming.
I did enjoy the merging of the stories, and even though the way they merged I did figure out beforehand, several times I doubted myself. Overall I really enjoyed The Fate of the Fallen, and I'm definitely reading the next book right away since I actually have it!