BTW, here, from the NYT, is about as short a description as you can get for this novel:
Two brothers, a Greek Catholic and a Roman Catholic priest, both connected to a controversial museum exhibit about the Shroud of Turin, are at the heart of a mystery set in the Vatican in 2004.
YOU MIGHT THINK, if you have been part of the shroud crowd for a few years, that the Fifth Gospel refers to the shroud. Well, it doesn’t; not in this book. So, now, you are going to have to read it.