In 2004, Ian Caldwell was the bestselling first-time author of The Rule of Four (Dial Press, 2004), written with his best friend and coauthor Dustin Thomason. Caldwell was about to marry his high school sweetheart, and he had a contract to write another book with Thomason. Seven years later he was a stalled writer with three young children, and his publisher was done waiting for the novel he seemed unable to complete. It was a classic publishing horror story.

But this story has a happy ending. Caldwell’s agent, Jennifer Joel of ICM, bucked up her distraught client and took his incomplete manuscript back into the marketplace. Jofie Ferrari-Adler of Simon & Schuster acquired it with enthusiasm. He calls The Fifth Gospel, the result of that manuscript, “a masterpiece,” and the book is due out in March.

“Jofie really shepherded this in the right direction,” Caldwell says, meeting with me in a Simon & Schuster conference room. “At a very early meeting he said, ‘Here’s the half we’ve got: I think some parts of this need to go here, I think you need to do a little more here. We’re going to spend the day doing note cards and storyboarding it, then you’re going to go home and get this done. You’re so close; I can feel it.’ ”

“At this point I did not feel it,” Caldwell continues. “I felt mired in what had become an albatross around my neck. But right after that meeting, I spent a couple of days at home and decided I had the blueprint of something good.”

The Fifth Gospel is a nail-biting thriller and a touching tale of fraternal devotion that smoothly incorporates some fairly arcane research. The protagonist, Father Alex, is an Eastern Catholic—a member of a church that follows its own rules, such as letting priests marry, while remaining under the pope’s direction. The plot of the book turns on a 2004 investigation into the authenticity of the Shroud of Turin, based on a close reading of the Diatessaron, a “fifth gospel” attempting to incorporate Matthew, Mark, Luke, and John into a single narrative that was ultimately rejected by the early Christian bishops.

The Rule of Four was also a thriller centered on an obscure text (the 15th-century Hypnerotomachia Poliphili), so it’s natural to assume that The Fifth Gospel is the novel Caldwell initially planned to write with Thomason. On the contrary, Caldwell says, “We were going to write a book centered on a 40-ft.-tall statue of Zeus that influenced Christian images of Jesus. It was before Rule of Four was published; we took a trip to Greece to research it, and we were at the same time pitching a show to ABC. We were in this period of creative flux when anything was possible.”

“Dusty really got sucked into the TV part of things [Thomason has since executive-produced several television series], and we decided that I was going to write the book by myself,” Caldwell says. “We had planned to write this book really quickly; Rule of Four took six years, and we vowed we would never do that again. If God has a sense of humor, he’s laughing about that. By the middle of 2006, I had a complete manuscript for the statue-of-Zeus story.”

There was one big problem, as Caldwell recalls: “It just didn’t speak to me. It came from the best intentions, but it was a hybrid. I brought the manuscript to my editor at Random House, Susan Kamil, and she asked, ‘If you had known you would be writing alone, would you have written this book?’ And I answered, ‘Honestly, no.’ ” According to Caldwell, Kamil responded by asking, “Then what are you doing?”

“We were in the middle of the editorial process,” Caldwell continues. “Random House was pushing full bore to get it out the door, but here [Kamil] was saying, ‘Put this aside, take some time, then tell me what you want to do.’ They didn’t even rewrite the contract.”

Caldwell had known for a while that the manuscript wasn’t really right, but he was reluctant to throw a wrench into an agreement on the basis of which he got married and started a family. “On the train home, I thought, ‘Susan’s on payroll there. If she has the guts to say, ‘Do what you need to do,’ how is it possible that I don’t have the courage to do it?”

It’s a relief that Caldwell’s story has a happy ending, because (insofar as a new acquaintance can tell) he’s a thoroughly nice guy who doesn’t have a bad word to say about anyone, not even the house that dropped him.

“Publishing is such a strange business,” Caldwell says. “Here I am in early 2011, sitting on a contract I signed in 2004 for a deliverable in 2006 that no one says is canceled yet, because it’s just understood that writers take forever. When Random House finally said they couldn’t keep waiting, I was paralyzed.”

Fortunately, even though it had taken much longer than expected, at this point Caldwell had written about half the text, with the Shroud of Turin plucked from a supporting role in the Zeus manuscript to star in a plot largely driven by intrigues within the Vatican.

The premise was compelling enough that Ferrari-Adler competed with several other houses to get the project for Simon & Schuster. Nonetheless, Caldwell was nervous. “The logical place for the story was the Vatican,” he explains, “and that was the last place I wanted to set a book! Dusty and I had been endlessly compared to Dan Brown, so I’m thinking, ‘Great: first, here’s my Da Vinci Code; now, here’s my Angels and Demons.’ I did not want to write this book, but it just kept insisting.”

It was simply irresistible, Caldwell found, to use the Vatican as the backdrop for a drama of two brothers on very different paths yet intensely loyal to each other, and the opportunity to give a priest legitimate marital problems was also appealing. “I was watching I, Claudius,” he remembers, “and that gave me the idea of an inside look at a family in power in the ultimate power structure. Mona and Peter [Father Alex’s wife and son] were always part of it, because I wanted to access those family feelings that come with having children. But in the beginning she just left ,and I didn’t have any plans to bring her back. Dusty and my agent both felt very strongly that she had to return, and they finally sold me on it.”

Along with Caldwell’s wife, Meredith, who is movingly thanked in The Fifth Gospel’s acknowledgments, Thomason provided crucial moral support during the manuscript’s darkest days. (He, too, was working on a novel then: 21.12, published by Dial in 2012.)

The lifelong pals may be coauthors again, Caldwell reveals. “Our 20th high school reunion was last month. Dusty came to town from L.A. and was over at our house for a while, and we just started brainstorming. It felt very natural—that’s all I can say about it!”

Wendy Smith writes frequently about literature and the performing arts for the American Scholar, where she is a contributing editor. She is a regular contributor of book reviews to the Washington Post, Los Angeles Times, Boston Globe, and the Daily Beast.