It's background is religious since it explores the Cathars, a gnostic Christian sect, and how they were brutally suppressed by more mainstream elements of the Roman Catholic Church in the 13th century. The book explores the situation by having a murder mystery / love story to engage our interest. Mosse does this by superimposing a modern storyline over one from the 13th century time period and running them in parallel. It works very well---but I do have a strong liking for that sorta thing.
One of my favorite series of books is by Katherine Kerr where she does a similar thing using ancient Celtic culture as her backdrop. The theme of love conquers time is just hard to resist. And besides, a religion that is dualistic and gnostic is frankly quite interesting albeit too strange to take seriously. At least by me.
Here's a paragraph on Catharism from Wiki:
This placed them [the Cathars] at odds with the Catholic Church in regarding material creation, on behalf of which Jesus had died, as intrinsically evil and implying that God, whose word had created the world in the beginning, was a usurper. Furthermore, as the Cathars saw matter as intrinsically evil, they denied that Jesus could become incarnate and still be the son of God. Cathars vehemently repudiated the significance of the crucifixion and the cross. In fact, to the Cathars, Rome's opulent and luxurious Church seemed a palpable embodiment and manifestation on Earth of Rex Mundi's sovereignty.
There's a lot more paragraphs there so feel free to give the Wiki link a click and find out more! :-)
Naturally this book invites comparison to The Da Vinci Code but there's a big difference: Mosse actually knows how to write. Just saying.