The Apocalypse Door

He's a priest. She's a nun. They fight crime.

Newark, New Jersey, isn't the city of Dis, but it could play the part on TV without spending a lot of time in rehearsals. By day, Newark's crowded and noisy and polluted, full of too many people going places too fast in pursuit of money or power or pleasure. By night, it's all that and dark as well, with danger waiting in the shadows to catch the unwary.
I'd just finished a job in Canada, checking out a report of Black Masses being celebrated, and was on a get-well tour in New York...

The Apocalypse Door is a novel by James D. Macdonald. A hybrid of spy thriller and urban fantasy, it concerns the actions of the Catholic Church's secret service, in the person of Father Peter Crossman (of the Inner Temple of The Knights Templar) and Sister Mary Magdalene (of the Special Action Executive of the Poor Clares), and a mission involving mushrooms, a brazen idol, and the imminent end of the world. A series of interludes tell the story of a man named Michael on a doomed op that connects to the main plot in ways both obvious and unexpected.

Averts Christianity Is Catholic to the extent it's not just window-dressing, or because the author doesn't know better; it's a deliberate attempt to create an intelligent and entertaining story within the Catholic world view, by an author who knows his subject. Everything the warrior priests and the fun nun with the gun do is justifiable within the Church's rules, and the fantastic elements are all drawn from Catholic lore. It should be emphasized that this doesn't mean the book isn't fun.

Peter Crossman and Mary Magdalene have also appeared in various short stories that originally appeared in Katherine Kurtz's Tales of the Knights Templar anthologies and have been collected in The Confessions of Peter Crossman.

Tropes used in The Apocalypse Door include:

Action Girl: Sister Mary Magdalene.
Beethoven Was an Alien Spy:
- The Knights Templar weren't disbanded in the 14th century, they just went underground; Peter Crossman and his colleagues are Templars. The legends surrounding them in the 14th century turn out to have an element of truth that's relevant to the plot.
- The Teutonic Knights are also involved.
Call Back: When Sister Mary Magdalene shows up, there's a call back to their first meeting in "Stealing God".
Celibate Hero: Peter Crossman, being a priest, and Mary Magdalene, being a nun. (It is established, though, that Crossman wasn't celibate before he became a priest.)
Confess in Confidence: A really twisted example.
Cross-Melting Aura:

Wherever the rosary touched the bag above the fungal glop, a thin trickle of grey smoke rose up into the still apartment air.
"Looks like that'll kill it," Simon said.
[...] It wasn't the fungus that was on fire after all. It was the rosary.

Doing In the Wizard: Subverted. Over the course of the novel, the protagonists piece together the evidence that the "demons" they're fighting are the spearhead of an extradimensional alien invasion -- but then, at the end, it's suggested that they really are demons.
Face Stealer: The mushrooms remove and wear the face of the person they're imitating. They also apparently take part of the brain, giving them access to the person's memories.
Fantastic Catholicism: Checks off "Fights The Supernatural" and "Is Full of Badasses".
First-Person Smartass: Peter Crossman.
Kill It with Fire: Turns out to be the most effective way of dealing with the threat.
Knowledge Broker: Dalrymple.
Meaningful Rename: "Peter Crossman" is not, as he puts it, the name he was given at Baptism. Probably this is also the case with his colleagues, and almost certainly also with Mary Magdalene.
Nun-Too-Holy: Sister Mary Magdalene of the Special Action Executive of the Poor Clares, "the fun nun with the gun", never actually strays beyond the bounds of her vows, but you wouldn't think it from the way she carries on.
Public Domain Artifact: The Head of Baphomet, an idol the historical Templars were accused of heretically worshipping.
Reformed Criminal: Implied to be common among Peter Crossman's colleagues. Crossman himself is a former Company man.
The Spymaster
- Michael's boss, called Joey.
- Peter Crossman's boss, called Prester John, is mentioned but does not appear in person.
Twist Ending: The way the story of Michael is ultimately tied in -- not the fact that Michael is Peter Crossman before he went straight, which is obvious very quickly, but the way it ties in specifically to Crossman's current case -- is such that even citing the specific trope would be a big spoiler.
You Said You Would Let Them Go: "I told you what I'd do to you if you didn't bring him back. I never made any promises about what would happen if you did."