“The Fifth Gospel” (Simon & Schuster)

The curator of a groundbreaking exhibit at the Vatican dies mysteriously hours before its premiere. Within hours, his research partner’s family becomes victim to a home invasion.

The second novel from Ian Caldwell, co-author of the best-selling “The Rule of Four,” kicks off at 90 mph and doesn’t slow down. Caldwell’s skill as a writer is evident in his ability to weave detailed descriptions of Biblical scripture, Catholic history and Vatican geography into the story while keeping the action going.

He has a lot of material to work with, having spent a decade on his follow-up to “The Rule of Four.” Co-written with Caldwell’s childhood friend Dustin Thomason, the murder mystery set at Princeton spent nearly a year on The New York Times best-seller list.

Caldwell thanks Thomason in his acknowledgements, noting that even before their novel was published, the two spent a week in Greece doing research that helped inspire “The Fifth Gospel.”

Caldwell’s new novel is set in the waning years of John Paul II’s papacy. The protagonist is Father Alex Andreou, a Greek Catholic priest who lives inside the Vatican with his 5-year-old son and who has been helping research the upcoming exhibit. The suspect in the curator’s death is Andreou’s brother, Simon, a Roman Catholic priest rising rapidly through the Vatican’s diplomatic ranks.

Greek Catholics observe the traditions of the Greek Orthodox Church while obeying the Roman Catholic pope. Unlike the Roman Catholic priests with whom they serve, Greek Catholic priests can marry and have families. A relatively small group, they are a remnant of the 1,000-year-old split between the Roman Catholic and Eastern Orthodox churches.

As Alex Andreou works to clear his brother of their friend’s murder, he learns that information crucial to bringing the churches together — or keeping them apart — could lie in a fifth gospel the curator discovered in the bowels of the Vatican library. The question is: Who would kill to keep it secret?

Andreou must retrace Catholic history and unravel convoluted scripture to solve the mystery. But Caldwell’s novel is more than a religious dissertation. He has created memorable characters with complex relationships, deep love and longstanding hurts. Both brothers carry the weight of having grown up in a Catholic minority inside the Vatican walls, with a Greek Catholic father who hoped but failed to bring the churches together. Both struggle with their own failings, as a father, brother or friend.

Ultimately, Caldwell’s novel is about faith — in God and in family. It ends as every Christian story does, with an act of forgiveness. (AP)