A lost gospel, a relic, and a dying pope's final wish send two brothers - both Vatican priests - on a quest to untangle Christianity's biggest mystery.

2004. As Pope John Paul II's reign enters its twilight, a mysterious exhibit is under construction at the Vatican Museums. A week before it is scheduled to open, its curator is murdered. The same night, a violent break-in rocks the home of the curator's research partner, Father Alex Andreou, a Greek Catholic priest who lives inside the Vatican with his five-year-old son. When the papal police fail to identify a suspect in either crime, Father Alex, desperate to keep his family safe, undertakes his own investigation.

To find the killer he must reconstruct the dead curator's secret: what the four Christian gospels - and a little-known, true-to-life fifth gospel known as the Diatessaron - reveal about the Church's most controversial holy relic. But just as he begins to understand the truth about his friend's death, and its consequences for the future of the world's two largest Christian Churches, Father Alex finds himself hunted down ...