BOB ABERNETHY, anchor: Now a look at the most important beliefs of the Church of Jesus Christ of Latter-day Saints -- the Mormons -- the fourth largest church in the country after Catholics, Southern Baptists, and United Methodists. Many non-Mormons admire Mormon family values, but Mormon beliefs are controversial, and that may be a big problem for former Massachusetts governor Mitt Romney, a prominent Mormon, in his campaign for the Republican nomination for president. Our special report today is from Bob Faw of NBC News.

BOB FAW: This is the image the Church of Jesus Christ of Latter-day Saints projects to the world: the majestic choir; the humanitarian efforts the church underwrites; earnest missionaries devoting 18 months to two years of their lives spreading Mormonism around the world; five-and-a-half-million Mormons in the United States, many prosperous and upbeat, respected for their lifestyle and family values, Mormons who neither smoke nor drink and who give 10 percent of their earnings to the church.

Dr. DALLIN OAKS (LDS Quorum of the Twelve Apostles): We make good citizens. There must be something good at the root when it bears that kind of fruit.

FAW: But to many Americans, and the Church is the first to admit this, Mormonism is shrouded in mystery and ignorance.

UNIDENTIFIED WOMAN #1: They get married and I think if the wife gets too old they get another one. I'm not sure (laughs).

UNIDENTIFIED MAN #1: I don't think they worship anybody as far as I know.

FAW: And to some evangelical Christians, the church is little more than a cult. Raised a Mormon, Sandra Tanner is now running a Salt Lake City bookstore which tries to debunk the faith.

SANDRA TANNER (Utah Lighthouse Ministry): You are talking about a very radical theology from standard Christianity. They're a heresy of Christianity.

FAW: Sentiments like that handicap presidential hopeful Governor Mitt Romney, a devout Mormon and leader in his church. In one poll, 35 percent of voters say they do not want a Mormon in the White House.

UNIDENTIFIED MAN # 2: Their belief system is based on things that I do not hold as true.

FAW: Their fear, although Romney denies this could happen, is that if a Mormon becomes president the country's needs would not come first. Dr. Phil Roberts, president of the Midwestern Baptist Theological Seminary:

Dr. ROBERTS (Midwestern Baptist Theological Seminary): Mitt Romney is a temple-worthy Mormon, and one of the conditions for being temple-worthy is that you have to swear allegiance to the Mormon president whom they believe can receive from God direct revelation. So Mitt Romney in a very real sense has an allegiance to a personality, and a person that in most religious circles is unprecedented.

FAW: So who's right? For starters, some Mormon beliefs do differ from traditional Christianity. Listen to the church's spiritual leader on the Nicene Creed:

President GORDON HINCKLEY (Church of Jesus Christ of Latter-day Saints, speaking to audience): Its basic elements are recited by most of the Christian faithful. Personally, I cannot understand it. To me the creed is confusing.

FAW: Other doctrines set them apart. Mormons believe Jesus returned to Earth and appeared to ancient Americans; that the Garden of Eden was in Missouri; that God was man and that men can be gods. Founder Joseph Smith said theirs is "the only true church" -- that all others were an "abomination." Dr. Dallin Oaks is one of the churches 12 Apostles.

Dr. OAKS: People say, "Where are you in relation to traditional Christianity?" I think our answer to that is we are traditional Christianity, because it is our message to the world that traditional Christianity had fallen away from the basic teachings of Jesus Christ, and it was necessary to have a restoration of the fullness of Christian doctrine and the original organization of the church with 12 apostles as Jesus established it.

SANDRA TANNER: Mormonism claims to be the only true church, not just a better church, not just another church. They claim to be the only true church and that in order to be right with God and go to the highest level of heaven you have to be Mormon, and for the Christian world that's, you know, fighting words.

FAW: Mormons have three books of Scripture other than the Bible - The Book of Mormon, which Joseph Smith said he translated from golden tablets revealed to him by God and the Angel Moroni. But it is in the books, Doctrine and Covenants, which contains the teachings of polygamy, and the Pearl of Great Price that includes the unique and, to some, non-Christian elements of the faith.

Dr. OAKS: We are different. That's the reason for our existence. The two main differences that separate us from other Christian faiths [are] some of the content of our doctrine and our belief in continuing revelation. Everything else follows from those two.

FAW: Dr. Dallin Oaks, a renowned legal scholar and former president of Brigham Young University, says Mormonism is part of mainstream Christianity. The primary difference, he says, is the Mormon concept of revelation and the Trinity.

Dr. OAKS: We believe in continuing revelation to guide us in the way we practice our faith, and we believe that God the Father and his son Jesus Christ and the Holy Ghost are separate and distinct individuals, and that causes us to look on our relationship with God and the mission of the Lord Jesus Christ in importantly different ways than other Christians.

FAW: Critics point out some of those differences.