In “The Fisherman’s Tomb,” former Supreme Court clerk John O’Neill tells the intriguing story of the discovery of the relics (bones) of the biblical Peter –– beneath St. Peter’s Basilica, on Vatican Hill in Rome, Italy.

St. Peter is a central figure in both the New Testament and in early Christianity. Jesus gave the keys of the Kingdom to St. Peter — and in Matthew 16:18, Jesus said, “You are Peter, and on this rock I will build my Church.”

From earliest times, Christian tradition held that St. Peter was executed in Rome by Emperor Nero and buried on Vatican Hill. However, until Pope Pius XII’s secret archaeological expedition — detailed in “The Fisherman’s Tomb” — the exact whereabouts of St. Peter’s remains were unknown.

In this true historical account, an archaeologist, an epigraphist (an expert in the study of ancient inscriptions), and a fierce early feminist named Margherita Guarducci all play a crucial role.

“You could write the greatest book of fiction in the world, and you could never invent a character like Margherita Guarducci,” O’Neill, a senior partner in an international law firm and former clerk to Supreme Court Judge William H. Rehnquist, told LifeZette. Following a passion he’s had for years, O’Neill has spent much of his life visiting and researching early Christian sites. (O’Neill’s first book, “Unfit for Command: Swift Boat Veterans Speak Out Against John Kerry,” was a New York Times number-one best-seller.)

“Since early childhood, I have been a student of Roman and early Christian history and archaeology,” O’Neill said in the press materials about the new book. “My love of these subjects and a restless soul have taken me to many major archaeological sites, from Christian catacombs south of Istanbul, to Crete and sites in North Africa.”

The story of the search for Peter’s final resting place that began nearly 80 years ago is rife with life-and-death dramas –– and the story of its author’s parallel search for personal faith is equally compelling.

Describing himself as a “doubting Thomas,” O’Neill, of Houston, Texas, explained that the process of writing this book had a profound impact on solidifying his own Christian beliefs.

“I wish I could say I believed in everything by faith,” O’Neill said. “But the truth is, something is wrong with me. I actually have to prove everything to myself. And terribly, I have to prove it with objective evidence.”

Prove it he did.

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“Those inscriptions renewed tremendously my faith, and to some degree, going through this book was a tremendous faith journey for me,” said O’Neill.

The inscriptions he’s talking about were secretly coded messages left by early Christians who risked their lives to leave a mark on history, quite literally.

Margherita Guarducci’s expertise and dogged determination more than half a century ago make it possible for Christians today — including “doubting Thomases” like O’Neill — to confidently proclaim that the theological basis of today’s Christianity is the same as it was for those of first-, second-, and third-century followers of Christ.

Though “The Fisherman’s Tomb” is a true story, it has all the elements of a great tale. It has memorable heroes and complex villains; fascinating, diverse, and unusual settings; and plenty of secrets and mysteries to satisfy readers, from Biblical scholars to skeptics of the faith.

Elements of “The Fisherman’s Tomb” (without spoiling it all) include a powerful but misguided Vatican insider; Nazis; popes; an anonymous American oil tycoon whose financing saved the lives of countless Italian Jews; some secretly coded ancient inscriptions; and even a female version of Indiana Jones.

“It’s a story that’s been totally suppressed. Virtually no one knows that Peter was found, that Pope Francis acknowledged the relics of Peter in front of a large square, and that there’s tremendous forensic evidence that that is, in fact, Peter,” said O’Neill.

Related: St. Peter and the ‘Greatest Speech of a Mortal’

About the motivation for his book, O’Neill commented, “I felt like it was a mission, a calling I was given to do with my life.”

And of his confidence in the conclusion regarding the discovery of Peter’s bones precisely where biblical accounts said they would be, he added, “I’m a lawyer. I’ve tried many cases. That’s a case I’ll try any time. I’ll win it all the time.”

“The Fisherman’s Tomb” is available here

Michele Blood is a Flemington, New Jersey-based freelance writer and a regular contributor to LifeZette.