Alex Bernier is a young American priest who, though he believes in God in his heart, is unsure of his own place. He is one of the last of the Carolingian order - priests that were given special powers by the church in olden times. They can identify and cast out demons and ghosts, etc.
Well, the head of the order dies under mysterious circumstances, and Alex (with Mara, the woman he once loved, a - apparently reformed - deranged escapee) journeys to Rome for the funeral. As the order's head had been excommunicated, he could not be buried in a holy grave; Alex decides to bury him anyway, as he knows that the priest was a good, holy man. Alex notices marks on the body over the heart; the marks are of teh Sin-eater, a man endowed with the power to take sins away from a person who the church would otherwise ignore, leaving them with a clean soul. The Sin-eater is then burdened with the other's sins, who can now die peacefullyand gain salvation.
ALex and his fellow Carolingian Father Thomas are charged by Cardinal Driscoll to seek out the Sin-eater. Thomas brings Alex to an underground anti-church who can help them. Sure enough, they're helped out, but they see some nasty things while they're there.
ALex, who has grown much closer to Mara (could she have been possessed when she committed here wrongs?) finds teh SIn-Eater. He is a lonely, but wealthy man who makes a living cleaning soulds of the wicked for profit. He extracts the sins - which manifest thesmelves as, um, ghostly kalimari - and eats them. But he has had enough, and desires that Alex take over. ALex refuses.
Alex and Mara have sex.
Alex comes back to find Mara near death. Alex does not want here to go to hell for whatever it is that she did so long ago, so he uses teh Sin-Eater power to cleanse her soul. But her soul is pretty clean... Alex has been tricked into becoming the next Sin-Eater.
(I forget the tie-in to the next step)
Cardinal Driscoll has been systematically conspiring to becomes the next pope... and he's also the head of the pagan cult! SHOCK! He's been working to become the next pope... a "warrior pope".
Driscoll is excommunicated and disgraced. He calls on ALex, the new Sin-Eater. But Alex, avenging Mara and his fellow priests, takes the sins of Father Driscoll and forces him to die with his sins.