(RNS) By naming devout, conservative Catholic U.S. Rep. Paul Ryan to be his running mate, former Gov. Mitt Romney, once a Mormon bishop, did more than ensure the U.S. will have a Catholic vice president in 2013.

He established the first Republican ticket without a Protestant since 1860, when Abraham Lincoln, who belonged to no church, chose Maine Sen. Hannibal Hamlin, a Unitarian, as his running mate, said Mark Silk, professor of religion and public life at Trinity College in Hartford, Conn.

Yet today's GOP ticket — two Christians who are neither evangelical nor mainline Protestants — isn't a major marker of social change, University of California history professor David Hollinger said.

For a real sign of the decline of American mainline Protestantism, Hollinger looks to the Protestant-free U.S. Supreme Court: six Catholics and three Jews. The Romney-Ryan ticket is well in line with today's wider, less brand-specific Christian culture, he said.

The number of Americans who identify with a Protestant denomination has been steadily slipping from more than 60 percent in the 1970s to 52 percent in 2010, said Duke University sociologist Mark Chaves, who tracks religion statistics in the national General Social Survey, conducted biannually by the National Opinion Research Center.

With this comes a shift in assumption about values, said Boston University professor Stephen Prothero, author of "The American Bible," which examines core civic, political, literary and religious texts of U.S. history and society.

"We can no longer assume when people speak of American values, they're speaking in terms of Protestants who dominated American religious and public life" since the nation's founding, Prothero said.

Besides the Supreme Court's makeup, he cited today's diversity in Congress, which has Catholics, Mormons, Jews, Buddhists and Muslims.

The Bible isn't every candidate's go-to text anymore. Prothero expects Vice President Joe Biden, a Catholic, and Ryan to argue over their different views of Catholic social teachings rather than stand on Gospel quotations.

Some evangelicals claim Ryan as one of their own, said David Brody, chief political correspondent for the Christian Broadcasting Network and author of a recent book, "The Teavangelicals."

Brody said that Ryan, as a "dedicated pro-life, true blue conservative," is a Tea Party evangelical, too, because he stands for the movement's agenda of "fiscal austerity and social conservatism."

Silk, who blogs at Spiritual Politics, points out, "I haven't heard any Protestants complaining that Romney didn't pick one of their own. The default mode is 'Christian' and Catholics are seen in the Christian tent."

Mormons see themselves as Christian; many scholars, but not all evangelicals, agree.

There's history to this. Joseph Smith, founder of the Church of Jesus Christ of Latter-day Saints in the early 1800s, "announced he had been called to set Christianity straight," said Robert Millet, professor of religion at Brigham Young University. Smith preached that the historic churches — their bishops, priests and pastors — had lost their divine authority and he offered a radically different vision of God, Jesus and the path to salvation.

Today, Millet said, "Mormons and Catholics are side by side with evangelicals in sharing very conservative beliefs in the traditional family, and in the idea that there are absolute truths and moral values."

The groups stand together in opposing abortion, same-sex marriage, and a provision in Obama's health care law that requires employers, including faith-based institutions, to provide or facilitate employee insurance coverage for contraception. When the U.S. Conference of Catholic Bishops called the rule an affront to religious liberty, Mormons were right with them, Millet said.

Ultimately, the symbolism of the Romney-Ryan ticket may be that it doesn't matter anymore which religion a candidate claims as long as he claims one, says Grant Wacker, professor of Christian history at Duke Divinity School.

"It would be much more significant if the candidates had no faith or called themselves agnostics or unbelievers," Wacker said. "That is still unimaginable in the USA today."

(Cathy Lynn Grossman writes for USA Today.)