Group leader Warren "Cubby" Culbertson did not tell the other four couples what he and his wife, Susan, had known for months: The governor was having an affair with a woman in Argentina.
When Jenny Sanford confronted her husband in January after finding a letter to "Maria" among his official papers, the governor turned to Culbertson. For nearly six months, Culbertson has been the first couple's spiritual counselor - and their secret keeper.
The Sanfords "passed" the Culbertsons' course with flying colors. A week later, Jenny Sanford asked her husband to leave their home.
In an interview with The Associated Press this weekend at his Columbia office, just blocks from the State House, Culbertson said he believed his friend when he said that this was his only marital transgression. He thinks Sanford was simply caught off guard by "the power of darkness."
Culbertson also thinks that the only thing holding his friends' marriage together right now is "their vow to God."
"Because it's not feelings - it's not emotions," Culbertson said, the smile fading from his tanned face. "For most Christians, at some point in your marriage, if you're married long enough, you do it because that's what we're called to do - out of obedience instead of out of passion. And I think that's where Mark and Jenny are right now."
The two men met in 1986, when Sanford was driving for Democratic gubernatorial candidate Phil Lader and Culbertson was a campaign volunteer. Culbertson, 51, owns a court reporting business and has been described as a pillar of the capital city's Christian community.
Culbertson helped found the Round Table, a Bible study that, according to a paper posted on the Web, offers men "a safe place to pose their questions, test their assumptions, and know that they will not be alone or stand out as a spiritual seeker." Sanford sought that spiritual refuge on at least a couple of occasions.
"God hates lawlessness and is tireless in His desire to dissuade man from his fascination with lawlessness," reads a paper titled "Cubby's Talks." "Our hearts are lions' dens of devouring lusts. Lawlessness torments righteous souls every day."
When Sanford made his tearful public confession in the lower lobby of the State House Wednesday, Culbertson was there. Sanford singled him out.
"I would consider him a spiritual giant," Sanford said, breathing heavily to stifle his sobs. "And an incredibly dear friend."
Culbertson was talking with Jenny Sanford earlier that day when he learned that his old friend had not been hiking the Appalachian Trail for six days, as he'd told staff. He had returned to Argentina.
During his 18-minute mea culpa, the governor made numerous references to "God's law" and the sin of self. They were straight out of "Cubby's Talks" and the CDs the Culbertsons used in their "boot camp."
When Sanford cited the example of King David's infidelity and fall during a meeting with his cabinet on Friday, he was also drawing on the Culbertsons' sessions.
"One of the quotes we use in our couples course is, 'You can choose your sins, but you can't choose your consequences,"' Culbertson said. "We used to use David as an example of that. Mark may be the 2009 version of a good example.
"Mark knew what David knew."
Some who watched Sanford's news conference felt he was insincere, playing to the Palmetto State's religious base to salvage his political future.
Janine Driver, a Washington, D.C.-based body language and deception detection expert, said Sanford showed more emotion when apologizing to Culbertson and longtime political aide Tom Davis than he did when speaking of his wife and four sons. She also believes he lied when a reporter asked if this was the first time he had been unfaithful.
"He answers the question before it's been asked," said Driver, who spent 15 years with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. "Then what does he do? He drops both his eyes and turns his head AWAY? ... That's what's called the cold shoulder. Because we face the core of our body toward people who usually address us."
Culbertson has asked his friend that same question several times over the years - not because he worried about it necessarily, but "just to kind of keep him accountable." It's a question occasionally asked of Culbertson.
"And I think it's something that men need to ask men who truly want to stay in line," he said.
Sanford told Culbertson he had never strayed before, and he "absolutely" believes him.
Culbertson knows that Sanford, like any man, has weaknesses. In Culbertson's view, Sanford's chief frailty was his inability to deviate from his own agenda in the political arena.
"Some guys are wired such that violating God's design in this area, of women, is a real challenge to them," he said. "That's not in his DNA. That's why it's such a surprise."
Even Sanford's political enemies would concede that much.
Will Folks, a former Sanford spokesman who has been excoriating his old boss in his political blog, said sex and romance "never seemed to be things that were on the governor's radar." Although he has since reported on two other alleged dalliances, Folks said this passionate love affair is "100 percent inconsistent with everything I ever saw of the man."
"I honestly thought the guy was asexual," Folks said. "I am not kidding."
In their course, the Culbertsons tell the couples - particularly the men - to avoid being put "in places that allow them to be more vulnerable in that area." Culbertson himself keeps his office door open when meeting with a woman and avoids dining alone with women.
In his confessional, Sanford said the relationship started as an "innocent" e-mail exchange, and Culbertson believes him.
Culbertson does not believe the other woman went out of her way to seduce a married man. When asked if he had met Maria, Culbertson paused, then looked up, an embarrassed smile creasing his face.
"I'm not going to comment," he said.
The Culbertsons have a waiting list for their boot camps, which they've held for about four years. They often hold the Bible study groups at their spacious colonial house overlooking Lake Katherine.
The Sanfords asked to take part and even offered the governor's mansion as a meeting place.
Culbertson said the course is "pretty intense." He quizzes prospective participants and asks them not to start the course if they don't intend to take it seriously.
"But they both committed to doing the work," he said, "and they both did all the work."
Besides Bible readings and prayer, the Culbertsons stage what they call a "date night," where spouses interview each other. Culbertson said the boot camp is "not a marriage course, but marriages benefit from it."
"Because it challenges husbands and wives to talk about things eternal that typically we won't do unless we're in a structured environment," he said. "So it's to challenge you - in your spiritual life, in your walk with the Lord."
Culbertson said media characterizations that Jenny Sanford kicked the governor out of their home are "pretty harsh." He noted that the children had just finished school for the summer, and that Sanford was coming off a grueling legislative session.
"There's a term we use in our couples boot camp: 'When emotions are high, discernment is low,"' he said. "And they both accepted that there was a lot of emotion where they were, obviously."
Years ago, Culbertson warned his friend about the demands and corrupting influences of public office.
When Sanford asked Culbertson to contribute to his 1994 congressional campaign, Culbertson refused. Sanford was "very offended," but Culbertson told his friend it's the rare man who can be an effective congressman and fulfill his biblical responsibilities as a husband and father.
Culbertson admits he was "crushed" to learn of his friend's betrayal.
"I hate to see anybody I love fall," he said. But he still loves Sanford, and he disagrees with those who would say the governor is unfit to lead the state.
Culbertson cannot say for sure whether any love survives between the Sanfords. But he would counsel them to stay together.
"I've seen God change hearts in ways that they can't imagine," he said. "And if I hadn't seen that over and over and over and over again, I would have no hope."