Transcript for Young Mormons on a Mission

You might find them going door to door in a neighborhood near ewe. Young men in shirts and ties, hoping to convert people to the church of jesus christ of latter day saints. One of the fastest growing religions in the world. This work is part of a demanding two-year right of passage, a journey once taken by mitt romney. Abc's bob woodruff brings us this look inside. hi, how are you doing to we're missionaries from the church -- Reporter: The two-year mission is a right of passage for most young mormon men. Have you ever seen missionaries in this neighborhood before? Reporter: There are 55,000 missionaries, serving around the world. And the church allowed our team rare access into their world for two days. Of door knocking, teaching -- called the vision of the tree of life. Reporter: And community service. I am a mormon. I'm a mormon. Reporter: Mormons have never been so visible. While mitt romney's presidential candidacy may have ushered in a mormon moment in this country, the church remains largely a mystery to those outside of the faith. Elder dustin. I'm sorry. Elder dustin. Elder? Yes, sir. Right here on my name tag, if that helps. It does. Reporter: Elder is a church term for the missionaries, though at age 21 and 20, they are anything but old. I like your hat. Reporter: They are serving on a mission in la mras, louisiana, a long way from home in utah. I didn't know too much about it. People would tell me, you're going to have jambalaya and gumbo. I'm like, what is that? I don't know what is that. Reporter: The church determines, they believe wh devine guidance, where missionaries are placed. Mitt romney served in bordeaux, france, where he has said he learned about rejection, trying to convince the french to give up wine. We're missionaries from the church of jesus christ -- we have our own religion, thank you. Zblf that's okay. Reporter: In the deep south, the missionaries often come across people with deeply rooted religious traditions. What church do you attend? Glory of god. Would it be all right if i left you with a little pamphlet? No, thank you. For some reason, they don't understand that we're trying to add to that faith in christ. Would you like to come in? Reporter: But then they mean yvonno, who immediately invites Do you have a faith in christ? Yes, I do. We would like to add to that and leave this with you. Okay. Because what's contained in that is the restoration of the gospel. Reporter: What do they mean about the gospel? A key difference between the latter day saints and other christians is that mormons believe that after the resurrection, jesus came to the ancient americas in a journey described in the book of mormon. While some continue to associate mormons with polygamy -- get out of here. Now. Reporter: And shows like "big love" add to the public's fascination with the topic, the church banned polygamy in 1890. Those some breakoff sects have continued the practice. The material that's most frequently asked for is copies of the book of mormon. Reporter: This is the baton rouge mission headquarters. That go okay? That was awesome. We pretty much just killed it, i guess. Reporter: President jim wall is a surrogate parent to the missionaries who are only allowed to call home twice a year, even though they pay most of their own way. Missionaries are required to turn in receipts for everything down to their gas miles. I think if I were a corporation, I'd hire a missionary, because they have such good training. Reporter: The missionaries work in pairs. Why do you keep these two young men together? They can't split apart except when they go to the bathroom. It's just for safety's sake. Somebody didn't wave a magic wand and take away the dumb. Once in awhile, they make bad choices. When you are with a companion, it's harder to make really bad choices. What do you think, skip? Reporter: The rules are strict. No el the on, no personal phone calls, no coffee or tea. Elder dustin looks nothing like his old i.D. Photo and they dress up even in the heat. We always wake up, try to be up by, before 6:30. In the name of jesus christ, amen. Reporter: By evening, they are ready for a home-cooked meal at the home of a browns, a family of local mormons, once converts themselves. They have gathered to welcome newcomer gloria adamson to the table. Is it likely you are going to be converted? Me and god are tight. Right now, the way I am. I suppose we could get a lot tighter. You know, if I repent and do whatever, but -- that's not going to happen any time soon. Reporter: Are there a lot of sins? That I've done? Reporter: Yeah. Absolutely. Absolutely. Reporter: And, she says, she's not ready to stop. Are you sorry for drinking? No. Reporter: You are going to be hard to be converted. Exactly. And I smoke, too. Reporter: You think you have a pretty good shot to bring her over? It's not really about that. As we taught her. We're there to build her faith in christ. I don't want our missionaries to think about a number. I don't want a person to think that they're a number. Reporter: But the church does track converts and their statistics show nearly 300,000 converts were baptized in 2011. All taught, at one point, by missionaries. And it is moments like this one that seem to excite the missionaries the most. Rick seems eager to hear their message. The book of mormon is a record of just that, the people that lived here in the ancient americas and how christ isn't just tior of the middle east, but he's the savior of the world. Reporter: They make an appointment to return. Good to meet you, rick. I've been a missionary for 20 months, knocked on countless doors, talked to countless people and the excitement doesn't go away. The excitement to tell people about what I know, what's brought me happiness and joy, it doesn't end. Reporter: I'm bob woodruff for "nightline" in laplace, louisiana.

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