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 Mr. Culbertson. For nearly six months, Mr. 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, Mrs. Sanford asked her husband to leave their home.
Mr. Culbertson said he believed his friend when he said that this was his only marital transgression. He thinks Mr. Sanford was simply caught off guard by "the power of darkness."
Mr. 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," Mr. 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."
Mr. Culbertson and Mr. Sanford met in 1986, when Mr. Sanford was driving for Democratic gubernatorial candidate Phil Lader and Mr. Culbertson was a campaign volunteer. Mr. Culbertson, 51, owns a court reporting business and has been described as a pillar of the capital city's Christian community.
Mr. 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." Mr. Sanford sought that spiritual refuge at least a few times.
When Mr. Sanford made his tearful public confession Wednesday, Mr. Culbertson was there.
During his 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 Mr. 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,' " Mr. Culbertson said. "We used to use David as an example of that. Mark may be the 2009 version of a good example."
Mr. Culbertson knows that Mr. Sanford, like any man, has weaknesses. In Mr. Culbertson's view, Mr. 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."
OLD STATE LAW MAKES AFFAIR ILLEGAL
COLUMBIA --- South Carolina Gov. Mark Sanford's admitted extramarital affair with a woman in Argentina could cost him his job and his marriage.
But, under an antiquated state law, it could also cost him up to $500 and a year in jail.
A state law that dates to at least 1880 says adultery is a crime, no matter where it occurred, and that adulterers "shall be severally punished." But constitutional attorney John Harrell of Charleston says it hasn't been used in decades, possibly not since 1907.
A spokeswoman for the State Law Enforcement Division says the agency can't waste limited money on trying to prosecute or arrest Mr. Sanford on such a charge. Jennifer Timmons notes there are more violent offenders to pursue.
-- Associated Press