With an increasing number of murders, you would think the theological aspect would get totally swallowed up by the mystery. But that's really not the case. Many pages are devoted to explaining heresies of the day, as well as recording debates between the monks about the nature of God and the devil. These parts of the book can be rather confusing (especially trying to keep track of all the different heretics), but they don't get in the way of the overarching narration. The most annoying thing I found was that the monks kept throwing entire sentences in Latin into their dialogue - with no translation! I understood what was going on anyway, but it got a bit on my nerves by the end.
This was not a short book, but it was a surprisingly fast read. The mystery kept the plot on track, and it was very difficult to put down in the last few chapters. If you get the right edition, there is a postscript from the author at the end explaining some of his thoughts as he wrote this book. In my opinion, here's the best line in the whole book: