Everyone knows the old saying about never discussing politics or religion at a dinner party. But sometimes both subjects deserve an airing, even if it makes dinner guests stare uncomfortably at their plates. Now may be one of those times.

In the current presidential campaign, former Massachusetts governor Mitt Romney, a lifelong Mormon, is undergoing scrutiny as to whether his religious faith might be an impediment to his being elected president and how it might affect his governance. His response, as on a recent appearance on “Larry King Live,” is usually to affirm a basic belief in God and pride in his faith, allude to John F. Kennedy in 1960 and then quickly get off the subject.

Now, I want to make clear that I give no quarter to religious bigotry. I was born, baptized and raised in a fundamentalist Pentecostal church (before they were popular and grew into today’s evangelical megachurches with televised services). My boyhood church, with its loud, frenetic worship style in which speaking in tongues was common, was viewed with disdain by both mainstream Protestants and Catholics. We were truly at the bottom of the denominational pecking order.

It was not uncommon for raw tomatoes or rotten eggs to be tossed through the church’s back door during worship services or for worshippers’ cars to be soaped with the term “Holy Roller” while we prayed inside. And I will never forget the snobbery and snide remarks directed at us because we were considered different, if not outright lunatics.

When John F. Kennedy was running for president in 1960, my family and I regularly attended church three to seven times a week. There was, admittedly and shamefully, a huge amount of anti-Catholic bias exhibited at the time by many fundamentalist Christians. Some of my fellow churchmen, in a shocking lack of Christian charity, referred to Catholics as “Catlickers” and the pope as “the old Poop.”

But in a political sense, the key rub back then was fundamentally governance: Would the pope, the monarchical and — to the faithful — infallible head of the foreign-based Roman Catholic Church (who some in my church actually saw as the Antichrist himself), give marching orders to a President Kennedy, thereby violating the sacred American principle of separation of church and state?

In today’s context, however, the most notable and troubling question about the Church of Jesus Christ of Latter-day Saints is not about church governance. Rather, it is about the Mormon Church’s official view — though recently soft-pedaled by the church hierarchy for obvious reasons — regarding the very validity of all other self-professed Christians, their faith traditions and their rituals.

The fundamental raison d’être of the Mormon Church is the core belief that no other Christian denomination whatsoever — whether Roman Catholic, Eastern Orthodox, Anglican, Lutheran, Methodist, Presbyterian, Congregationalist, Assemblies of God, Baptist, Anabaptist, you name it — is valid. In fact, Mormons assert that there was no legitimate Christian church whatsoever, and therefore no legitimate Christians, virtually from the time the last remaining apostles died off at the end of the first century until the founding of the Mormon religion in 1830 by the self-proclaimed latter-day prophet Joseph Smith. They refer to this nearly 1,800-year interregnum as the “Great Apostasy,” which pretty much speaks for itself.

Smith was born in Vermont, one of eight children of a struggling farmer and occasional schoolteacher. By 1816, the family had moved to Palmyra, N.Y. Smith’s father was reputedly religious but unchurched. In 1820, young Joseph was pondering which of the most prominent Protestant denominations of that day to join. Smith’s mother, two brothers and a sister had become Presbyterians. According to an account he wrote 18 years later, the 14-year-old Smith repaired to the woods to seek God’s guidance as to which church merited his allegiance.

He reported a vision in the forest in which both God the Father and Jesus the Son appeared before him. Smith asked the spectral heavenly presences which denomination he should join. Here, in Smith’s own words, in “Pearl of Great Price,” a writing published by the Mormon Church, is the response he claims to have received directly from the mouth of God (my comments added in brackets):

“I was answered that I must join none of them, for they were all wrong; and the Personage who addressed me said that all their creeds were an abomination [apparently, including the Nicene Creed and the Apostles’ Creed, the two most common summations of Christian belief] in His sight; that those professors [those who professed those creeds] were all corrupt; that ‘they draw near to me with their lips, but their hearts are far from me, they teach for doctrines the commandments of men.’”

All Christian creeds an abomination? Every follower of an existing denomination corrupt? So much for ecumenical spirit — or even basic religious tolerance. This means, in Mormon doctrine, that the post-apostolic Roman Catholic Church has been apostate during its entire 2,000-year history; the Eastern Orthodox churches that split from Rome in 1054 also were and remain illegitimate; even the various denominations founded by the great Protestant reformers in the 16th century — Martin Luther, Thomas Cranmer, John Calvin, Huldrych Zwingli, Jan Hus — are all null and void.

How does this hard-line theology play out in real life? Let’s take baptism, the nearly universally accepted, biblically mandated rite of initiation into the Christian faith (Jesus himself was baptized by John the Baptist). Even the Roman Catholic Church acknowledges the validity of baptisms carried out by other faiths if they are performed using water and the Trinitarian formula (“in the name of the Father, and of the Son, and of the Holy Spirit”). The overwhelming majority of Christian traditions also believe that emergency baptisms are valid even when administered by a layperson, say at a moment of great peril or impending death, when no ordained clergy are available.

But Mormons believe that no baptism performed between the extinction of the original 12 apostles and the founding of Mormonism in the 19th century was valid, and that even today, only holders of the Mormon priesthood can perform an efficacious baptism. The ubiquitous Mormon missionaries who circle the globe — Romney himself was one, in France, while he was in college — are trained specifically to proselytize and convert members of other Christian faiths and bring about their rebaptism into the Mormon Church.

What about all those billions of putative Christian souls through the last two millenniums who thought they were saved via baptism in a non-Mormon faith but were unwittingly lost? Mormons hold that only a posthumous proxy or “vicarious” baptism by a qualified Mormon practitioner, in temple ceremonies off-limits to non-Mormons, will ensure they end up in paradise. Hence the church’s famous focus on extensive genealogical research.

This “our way or no way” approach is pretty much unique to Mormons in this day and age. No doubt there remain some hard-liners in the Vatican who still believe Roman Catholicism to be the only “one true church.” But that attitude is most certainly not shared by rank-and-file Catholics in this country, millions of whom have abandoned the church of their upbringing and many of whom have defected to evangelical brands of Christianity. While there are still no doubt some anti-Semitic Christians who believe God doesn’t hear the prayers of Jews, most American Christians today believe that Jews are God’s chosen people and that they will be in heaven.

I wager that this exclusionary Mormon theology, once widely revealed, will come as a shock to most U.S. Christians, regardless of their particular denominational preference or worship style. This is a country where ecumenism and church shopping have been de rigueur for decades. Christians in our pluralistic society are accustomed to local interfaith councils, interdenominational prayer breakfasts, Shrove Tuesday pancake suppers shared by various denominations, fall harvest dinners that rotate among the church halls of different local denominations. They are accustomed to Lutherans and Presbyterians sharing their church buildings with Korean Methodists and to Episcopalians and Lutherans exchanging ordained clergy.

I believe that Mormon doctrine on this point, once made plain, will prove equally offensive to modern-day, ecumenical-minded Christians around the world, who in recent times have:

Witnessed Pope John Paul II as the first pope to visit a synagogue, meeting in Rome with the grand rabbi and asking for forgiveness from Jews for the persecution and forced conversions of previous centuries. He also called on the grand mufti of Jerusalem on the Temple Mount.

Observed several archbishops of Canterbury, spiritual leaders of the church formed by Henry VIII when he unilaterally broke with Rome in the 16th century (and was excommunicated by the pope for his trouble), make pilgrimages to the Vatican to meet with various pontiffs over the past few decades.

Watched Pope Benedict XVI pray facing Mecca with an imam at Istanbul’s famed Blue Mosque and meet with the ecumenical patriarch of Eastern Orthodoxy. (For those who don’t know their religious history, after the Great Schism of the Western and Eastern churches in 1054, the pope and patriarch had excommunicated each other.)

My own pilgrim’s progress is not dissimilar to that of many Americans in recent times. I have peregrinated from Holy Roller to High-Church Anglican, with rest stops in between at Baptist, Lutheran and even Greek Orthodox churches. In my immediate family, one side going back was French-Canadian Roman Catholic and the other mainly Midwestern Seventh-day Adventist. (My in-laws, incidentally, are Buddhist.) Yet despite the fact that my personal worship preferences are strong, I — like the overwhelming majority of most practicing Christians — would never question the Christianity or certainly the baptism of those in other denominations.

I’ve met Mitt Romney, and he is clearly an intelligent, articulate, impressive and accomplished man. He was elected a Republican governor of one of the most reliably Democratic states in America — the only one that voted for George McGovern in 1972 — which was no small feat. His leadership of the once-flailing 2002 Winter Olympics in Salt Lake City drew almost universal praise. His father, George Romney, was a decent man and an honorable governor of Michigan, as well as a one-time Republican presidential candidate. His mother once ran for the U.S. Senate.

I would never doubt the sincerity of Romney’s religious convictions, his charitable views toward humankind in general or the strong family values that motivate devout Mormons like him. But none of that is the issue at hand.

In many media interviews with Romney over the last several months, reporters and interlocutors have tended to focus on the more exotic things such as the Mormon history of polygamy, the strange origin of the Book of Mormon and the belief unique to Mormons that Jesus Christ personally ministered to ancient Indian tribes in North America after his resurrection — and will return in triumph to the state of Missouri. These inquiries, however, while perhaps titillating, are mainly off the mark.

The far more critical and basic question is: Does Romney’s brand of faith and membership in the Church of Latter-day Saints require that he question or dismiss the validity of the Christian tradition, and the efficacy of baptism into their faith, of every non-Mormon adherent of Christianity who has ever lived since the end of the apostolic era? And does he?

In the last presidential election, I was a senior adviser to Sen. Joe Lieberman of Connecticut, who in 2000 was the first Orthodox Jew to be on a major-party ticket and in 2004 was the first to run for president. During his campaign, Lieberman was often asked this question by sincere, well-meaning Christians: As a practicing Orthodox Jew who clearly does not believe in Jesus Christ as the Messiah, would he allow the traditional White House Christmas tree in the Blue Room? (The answer? Of course, because it is a long-standing national tradition and the tree is not a religious symbol in and of itself.)

If Lieberman was queried about such a considerably less weighty matter pertaining to his faith, then I submit it is neither unfair nor inappropriate to expect a would-be President Romney to publicly state whether he personally believes, as does his church, that every non-Mormon Christian he would govern was invalidly baptized in an illegitimate church.