Summary of Ian Caldwell's The Fifth Gospel is a literary thriller that takes place in 2004 at the Vatican, the chief residence of the Catholic Pope in Italy. The novel follows the aftermath of Vatican museum curator, Ugolino Nogara's, murder.

Brothers, Alex and Simon Andreou, are both priests. Alex is a Greek Orthodox priest and Simon is a Roman Catholic priest as well as Vatican diplomat in the Secretariat. Their opposing religious affiliations place the brothers on opposite sides of a centuries old feud between the two churches. 

Alex is married, but his wife, Mona, left him and their son, Peter, when the boy was a baby.

Ugolino Nogara is planning an art exhibit in the Vatican museum. A week before the exhibit is to open, Ugo is found dead. The discovery is reported by both Alex and Simon. That same night, a man breaks into Alex's apartment. Peter and the nun who cares for him hide and escape unharmed...