When his friend Andrei is mysteriously killed on a train on his way back from Rome, Father Nil, a Benedictine who teaches the Gospel of St John to novices, decides to conduct his own investigation. The dead priest possessed proof of the existence of a thirteenth apostle and an epistle stating that Jesus was nothing more than an inspired prophet, not the Son of God - two things that would spell great danger for the Church. Father Nil then discovers a previously unpublished account of the origins of Christianity. It tells of the Nazoreans - a community excluded from the official Church by Peter and Paul - who appear to have thrived until the 7th century, playing an important role in the birth of Islam. While he pushes ahead with his investigation, the Pope's advisors, rival factions and secret societies are trying, by any means, to lay their hands on the priest's findings. From the Mossad to Fatah, everyone seems to have a very good reason to keep the thirteenth apostle a secret...