Thrillers are popular, so you need a tale with skullduggery and underhanded dealings. Having the main characters in clerical garb can't hurt. Dan Brown made a fortune off the clergy so why shouldn't you indulge?
Using a real life incident makes the plot evolution a bit easier because it's already been done. All you have to do is flesh out the narrative arc and you're on your way.
Your novel opens in Italy, in a charming town in the Roman hills called Arricia. They are famous for their wild boar pasta sauce, a fact you might want to insert at some point to add local color to your story. Ladle that on a pile of polenta on a cold day, sip a glass of red wine, and you could hike for hours in the bitterest cold and never feel the chill.
But that's the fluff that goes in between the action.
Back to your opening in Arricia. Two priests arrive on the train and walk to town, where they have been summoned by someone claiming the priests have been engaging in some financial shenanigans. This happens on the same day that their fellow clerics of the Order of St. Camillus are meeting in Rome to elect their new superior general, a man who will hold great financial power once he is elected because he will be in charge of a collection of hospitals that buy expensive supplies. The contracts are lucrative and the temptation to influence the election is great.
|A vote for Father Salvatore is a vote for corruption|
While the priests are grilled for hours, your scene shifts to the Rome conclave where a seemingly innocuous accountant appears, to whisper in the ear of the leading candidate who happens to be Italian. The other priest in the running is an Irishman, capable enough but not "one of us" as the accountant might put it.
As the story unfolds, you insert bits of backstory that show the accountant is not the least bit harmless. Indeed not. He has been implicated in other schemes, such as misuse of public funds and acting as the bagman for Silvio Berlusconi in a plot to hide bribes. Paolo Oliverio, the accountant, is trying to fix the election so that he can manipulate the hospital contracts and get kickbacks from those he selects as vendors to the network of Camillan hospitals in the south of Italy.
Your protagonist could be an undercover carabinieri or a priest assigned by Pope Francis who is responding to the complaints of those two Camillan priests who were mysteriously detained in Arricia while the vote was being taken in Rome. They would have voted for the Irishman, you see, and their detention was all part of the plot to get Father Renato Salvatore elected.
It's your novel so you can go where you like, but if you're following the real escapades of the Order of St. Camillus, you'll have Father Salvatore arrested along with four other priests and the accountant. The Pope could make an appearance in your novel, if you felt so inclined, perhaps meeting secretly with the protagonist who uncovered the evil in the heart of a man of God. Of course, you'd want the Pope to select the Irishman to the post of superior general while Father Salvatore rots in prison, and that would be a fine place to end your book.
In the middle you'll put in a Vespa chase or two through Rome, and perhaps a foot chase through the hills, around the volcanic lakes near Arricia. Don't forget a few red herrings to throw your reader off track so they can't guess right off who is conniving and who is innocent.
Now go sit down and write.