Chelsea Clinton, a Methodist, and Marc Mezvinsky, a Conservative Jew, had their very private wedding last Saturday. But the public may not be done peering through the shrubbery at their lives.

Like it or not, the famous bride and groom will continue to be the focus of scrutiny for their religiously mixed marriage — a category that’s growing rapidly among U.S. couples.

Two decades ago, 25 percent of U.S. couples didn’t share the same faith. That was up to 31 percent by 2006 to 2008, according to the General Social Survey by the National Opinion Research Center at the University of Chicago. The number was even higher, 37 percent, in the 2008 U.S. Religious Landscape Survey by the Pew Forum on Religion & Public Life.

Both surveys included people who crossed major traditions, such as Jewish-Protestant; believers married to the unaffiliated; and Protestants of different denominations, such as former President Clinton, a Baptist, and Secretary of State Hillary Clinton, a Methodist.

FORSAKING TRADITIONS

For those of nominal faith, this is no big deal. “Everybody party” is a popular way to avoid doctrinal conflicts, however thin on theology. But for those who hold deep but different faiths, life-cycle decisions will loom, from baptism (No? Yes? Whose church?) to burial (Can you rest in sacred ground of another faith?).

Every rite of passage, sacred ritual and major holy day will require negotiation: First Communion? Bar or bat mitzvah? Passover Seder, Easter vigil or Eid Al-Fitr feast to break Islam’s Ramadan fast?

Looking on: Parents and clergy who fear that distinctive beliefs, sacred rituals and centuries-old cultural traditions will be diluted or discarded.

Saqib Ali, who is Muslim, and his Christian wife, Susan, eloped to a nearby mosque for their Muslim wedding 11 years ago, since “neither set of parents was interested in our getting married,” Ali said.

“A few months later, when everyone settled down, we had a Christian ceremony and a big reception,” he said. They are rearing their two young daughters in Islam, while still going to Susan’s Presbyterian USA church on Christmas and Easter.

“I don’t try to force my religion on her, and she doesn’t force hers on me,” said Ali, of Gaithersburg, Md.

“If one of you is more rigidly religious, no matter what the religion is, it’s a whole lot harder,” said Cathy Dee of Fort Wayne, Ind., a Methodist who married a committed Catholic 35 years ago. Although their two children grew up Catholic, both now attend Protestant church with Dee’s grandchildren.

That’s the kind of drift that worries Gerald Harris, a retired Southern Baptist pastor. He objects most strongly when teachings or traditions are blended like a margarita or simply forsaken.

‘ABSOLUTE TRUTH’ AT STAKE

Harris, editor of a century-old magazine, The Christian Index, defines “interfaith” unions very sharply: believers who have accepted Jesus as their personal savior and anyone — Christian or any other religion — who has not. As a pastor, he declined to wed Christians to unbelievers. At funerals for unbelievers, Harris would gently tell mourners that true comfort was with Christ.

But, he said, “it is going to be increasingly difficult for people who hold to their faith firmly and strongly. The idea of absolute truth is what is at stake here.”

When retired United Methodist Bishop Kenneth Carder did premarital counseling, he found “many religious folks don’t know their own tradition — Methodist or Catholic or Baptist.”

That makes it hard to foresee if your faith will meld or conflict with your partner’s, says Carder, who teaches at Duke Divinity School. His goal was to “help Methodists be Methodists, Christians be Christians and both partners live with integrity for both traditions.”

The intermarriage trend is probably here to stay. The Pew survey found that 20 percent of Protestants have married outside their tradition; the 2001 American Religious Identification Survey, which included smaller groups, found the intermarriage rate was 39 percent for Muslims, 27 percent for Jews, 23 percent for Catholics and 12 percent for Mormons.

LOSSES AND ACCEPTANCE

A growing number of Catholics are skipping church weddings no matter whom they marry, says Mark Gray, a research analyst at the Center for Applied Research in the Apostolate, which tracks Catholic statistics.

He says the annual rate of marriages at which a priest officiated was down from 4.3 per 1,000 Catholics in 1999 to 2.6 per 1,000 in 2009.

Even so, Eric Andrews, the priest in the BustedHalo.com website video series, “The Priest, the Princess and the Search for the Perfect Wedding,” says he has seen countless matchups with a Catholic marrying a Jew, a Muslim, a Protestant or an atheist. It’s only later, he said, that “the fireworks start.”

The church requires that a Catholic promise to rear any children as Catholics, “to the best of his or her ability.”

But in reality, Andrews says, most mixed couples go one of three ways: They toss the fun parts of faith at the kids and let them decide a religious path later, or one parent takes the lead, or they shift to secular lives and let it drop.

The website InterfaithFamily.com offers DVDs for a “Love and Religion” course created by Marion Usher, a marriage and family counselor who runs workshops for interfaith couples at the Jewish Community Center in Washington, D.C. DVD sales soared after Usher began offering advice online timed with the Clinton-Mezvinsky wedding.

She suggests couples pick a “lead religion” and respect the other one for its cultural heritage. No matter which path they take, Usher said, “they must realize they face losses, acceptances and compromises.”