(CNN) There's a strange resemblance between religious conversion stories and weight-loss ads: both rely on astutely edited "before" and "after" images.

To sell slimming products, the camera first shows a man facing forward, flaunting his flabby gut and lumpy love handles. In the "after" shots, the camera is angled to the side, highlighting a newly narrowed midriff.

The goal of the illusion isn't to just make the man look better, of course; it's to make viewers believe that a product has the miraculous power to turn blubber into brawn.

As anyone who has spent time in a church, synagogue, mosque, temple, sangha, AA meeting or prison knows, a similar effect can arise in religious conversion stories. The "before" pictures, in particular, tend to darken. The snares of sin sharpen, the descent into depravity deepens.

Often enough, eye-roll-worthy embellishments are accepted, even expected. What's a little stretch when you're winning souls for Christ, or escaping bad karma? But sometimes converts' zeal can get the better of them.

The questionable content of conversion stories came under scrutiny last week with reports about Ben Carson, the GOP presidential candidate who credits God with his remarkable rise from poverty to renown.

Photos: Ben Carson's career in politics Photos: Ben Carson's career in politics Ben Carson attends the National Action Network (NAN) national convention at the Sheraton New York Times Square Hotel on April 8, 2015, in New York City. Hide Caption 1 of 13 Photos: Ben Carson's career in politics Carson speaks during the 41st annual Conservative Political Action Conference (CPAC) at the Gaylord International Hotel and Conference Center on March 8, 2014, in National Harbor, Maryland. Hide Caption 2 of 13 Photos: Ben Carson's career in politics Carson speaks to guests at the Iowa Freedom Summit on January 24, 2015, in Des Moines, Iowa. Hide Caption 3 of 13 Photos: Ben Carson's career in politics Carson is surrounded by supporters as he waits to be interviewed at the annual Conservative Political Action Conference (CPAC) at National Harbor, Maryland, outside Washington on February 26, 2015. Hide Caption 4 of 13 Photos: Ben Carson's career in politics Carson speaks at the South Carolina Tea Party Coalition convention on January 18, 2015, in Myrtle Beach, South Carolina. A variety of conservative presidential hopefuls spoke at the gathering on the second day of a three-day event. Hide Caption 5 of 13 Photos: Ben Carson's career in politics Carson delivers the keynote address at the Wake Up America gala event on September 5, 2014, in Scottsdale, Arizona. Hide Caption 6 of 13 Photos: Ben Carson's career in politics Carson speaks during the 41st annual Conservative Political Action Conference (CPAC) at the Gaylord International Hotel and Conference Center on March 8, 2014, in National Harbor, Maryland. Hide Caption 7 of 13 Photos: Ben Carson's career in politics Carson speaks during the National Prayer Breakfast at the Washington Hilton on February 7, 2013, in Washington. Hide Caption 8 of 13 Photos: Ben Carson's career in politics Honoree and director of pediatric neurosurgery at Johns Hopkins University, Carson poses with actor James Pickens Jr. at the Jackie Robinson Foundation Annual Awards Dinner on March 16, 2009, in New York City. Hide Caption 9 of 13 Photos: Ben Carson's career in politics Before his jump into conservative politics, Carson was known for his work as a neurosurgeon. Carson was awarded the Presidential Medal of Freedom by then-President George W. Bush on June 19, 2008. At that time, he was the director of pediatric surgery at Johns Hopkins Hospital in Baltimore, Maryland. Hide Caption 10 of 13 Photos: Ben Carson's career in politics In a story that garnered international attention, Carson was ready to separate a pair of 10-year-old Indian girls, Saba and Farah Shakeel, who are joined at the head in New Delhi, India. Here, he addresses a press conference at the Indraprashtra Apollo Hospital on October 4, 2005. Hide Caption 11 of 13 Photos: Ben Carson's career in politics Carson and a team of 20 specialists approved the procedure after studying the girls' brains; however, their parents were worried about their daughters' lives and did not give doctors permission to operate. The surgery did not happen. Hide Caption 12 of 13 Photos: Ben Carson's career in politics Carson observes the start of neurosurgery proceedings at the Raffles Hospital in Singapore on July 6, 2003. Carson and Dr. Keith Goh, left, performed a complex operation that was unsuccessful to separate 29-year-old twins Ladan And Laleh Bijani, who were joined at the head. Hide Caption 13 of 13

To some extent, Carson's campaign, as well as his longstanding popularity among conservative Christians , is built upon the stories he tells about himself and his relationship with God, including the narrative of his dramatic conversion from a violent young man to spiritual tranquility.

As a teen growing up in Detroit, Carson says, he reacted angrily to perceived slights, trying to bash his mother with a hammer after an argument about pants and attempting to stab a close friend -- an unnamed relative, he now says -- over a radio.

If those incidents seem to contradict Carson's sedate public persona, that's the point. Only God could change a man that much, he says, telling the story of how, after trying to knife his relative, he prayed in a bathroom for three hours, asking the Almighty to tame his raging temper.

"When I came out of that bathroom I was a different person," Carson says . "I had really had an experience with the Lord."

In a story published last Thursday, though, childhood friends told CNN they don't recall any of the vicious fights Carson has detailed. Neither do such stories jibe with their recollection of him as a bookish, quiet kid, his friends said.

Carson retorted that CNN had "talked to the wrong people" and said the person he almost stabbed does not want to talk to the media, which he accused of conducting a "witch hunt."

"The burden of proof is not going to be on me to corroborate everything I have ever talked about in my life," Carson told journalists

Whether or not every detail can be corroborated, Carson's conversion story has long plucked a powerful chord among evangelicals, said Ted Olsen, managing editor of the magazine Christianity Today.

It follows a script familiar to many born-again Christians, Olsen said: "I had an issue in my life. I was going down the wrong path. I turned to God and surrendered my bad behavior, and (God) took it from me."

"That kind of story rings true among a lot of evangelicals."

(Carson also embodies a common evangelical fantasy, Olsen added: The man so peaceful that strangers ask for his secret, opening an opportunity to evangelize about Jesus. It has the added benefit of deflecting attacks from people like Donald Trump who say Carson is too chill to be commander in chief.)

In Carson's church, Seventh-day Adventism, personal testimonies are not particularly common, though they are offered occasionally during worship on Saturdays, said Dan Weber, a spokesman for the denomination.

But since the early days of the evangelical movement, nearly all born-again Christians have been expected to know the exact hour they "made a decision for Christ," in the memorable words of evangelist Billy Graham.

Without a conversion -- and a coherent story about it -- your commitment to the church could be considered suspect, said Bruce Hindmarsh, a professor of spiritual theology at Regent College and author of "The Evangelical Conversion Narrative."

"On the whole, people are trying to bear witness to the fact that God has broken into their lives, that God still acts in the present world," Hindmarsh said. "It's not supposed to be a story about yourself. It's supposed to be about God."

The most memorable conversion stories are often the "stickiest," to use Malcolm Gladwell's popular phrase: easy to understand but surprising and bit counterintuitive. The persecutor of Christians, like St. Paul, who became one of the faith's most ardent apostles. The Oxford atheist, like C.S. Lewis, who turned Christian apologist. The raging boy, like Carson, who matures into an elder statesman.

But the apotheosis of autobiography can have its drawbacks, many evangelicals say: the temptation to tell a story that sounds, and might be, too good to be true.

Among the more notorious examples of salvation slipping into showmanship was Mike Warnke, a Christian comedian and evangelist who claimed to have converted after a violent and scandalous sojourn as a high priest in Satanism.

"Exaggeration did creep into some of my stories," Warnke later admitted to an Oklahoma newspaper in 2000 , "but my testimony is still my testimony."

More recently, several evangelicals have been accused of telling tall tales about their Muslim past , with some suggesting they were involved in terrorism before converting to Christianity. Five years ago, for example, Liberty University, one of the country's largest evangelical schools, removed its seminary dean, Ergun Caner , after finding that he had "made factual statements that are self-contradictory."

"Testimony envy may be part of any community," Olsen wrote in an editorial after Caner's removal, "but we evangelicals seem to have a particular bent toward narrative one-upmanship."

Carson is far from the first presidential candidate to face scrutiny about his conversion story. In his campaign biography, George W. Bush recounts a private walk with Billy Graham in Maine in 1985. The aging evangelist "planted a mustard seed in my soul," Bush later wrote, leading him to quit the bottle and get serious about God.

Invoking Graham, a man revered by millions of evangelicals, was politically astute. The story made the rounds in evangelical publications like Charisma magazine . But asked by reporters for his own recollections, Graham admitted that he had little memory of the famous "walk." And other pastors in Texas suggested that Bush was well on his way to redemption years before the aging evangelist came to Kennebunkport.

Collapsing multiple stories into one anecdote and giving a famous preacher more credit than he deserves aren't considered cardinal sins in American politics. In the end, condensing his faith journey didn't cost Bush much, said Randall Balmer, a professor of history at Dartmouth College and author of "God in the White House."

For Carson, the stakes are quite a bit higher, Balmer said.

"His whole campaign is based upon his narrative, because he has no political experience and no record of public service," said Balmer. "If it is undermined or called into question, it's going to be tough for Carson."

Evangelicals experts agreed. A little fuzzing around memory's edges is expected. Inventing stories about a violent past would be far less forgivable.

"His selling point is that he is a man of integrity, said Michael Duduit, dean of the College of Christian Studies at Anderson University in South Carolina. "If it turns out he made up stories out of whole cloth, that would be very disconcerting."

For 30 years, Duduit -- an expert on preaching -- has been schooling evangelical pastors on how to make a story stick while sticking to the facts.

"I tell them it's OK to tell a story. Jesus told stories," Duduit said. "Just don't add details that didn't happen. If you do, four or five millennials in the congregation will start Googling you before the sermon is even over."