Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
…God spared not the angles that sinned, / but cast them down to hell / and delivered them into chains of darkness / to be reserved unto judgement… (2 Peter 2:4)
What's up with the epigraph?
The epigraph comes from 2 Peter, a book of the New Testament, and it explains how angels who sinned were banished from heaven.
The concept of the fallen angel works as the base for the fantasy element of the story, and the story continues to make occasional reference to biblical figures and events, while also doing a lot of world-building around The Book of Enoch. Enoch is said to have been the great-grandfather of Noah (you know, the guy with the ark and all the animals).
Despite this epigraph, however, Hush, Hush is not about religion. For instance, when Nora finds out that Patch is a fallen angel, she doesn't ask him about heaven or God. Instead, the angels and their backstories seem to be treated more like secular fantasy figures than religious ones. So why reference the bible at all in the epigraph?
In addition to giving some hints about the subject and themes of the book, biblical quotes quickly raise the stakes, throwing Hush, Hush immediately into a long literary tradition and linking it to the timeless, epic conflicts and themes in the bible such as good vs. evil, human vs. supernatural/divine, sin, sex, fall, and redemption. If you want to start your book with a bang, it's hard to go wrong with something from the bible.
To dig into this a bit deeper, swing by the "Writing Style" section.