Limestone gleamed in the spring sun as Elder Paul Sinclair patrolled the grounds of a project that has consumed much of his life these last few years. A square-jawed former linebacker at Brigham Young University, labor lawyer at the downtown law firm Ice Miller, and high-ranking official in the Church of Jesus Christ of Latter-Day Saints, Sinclair has presided over the construction of the Church’s first temple in Indiana, a structure situated on a plush 50-acre parcel not far from the tony McMansions of Carmel. At the property’s entrance near the intersection of Spring Mill Road and 116th Street, a churlish private security guard hired by the Church stood sentinel outside of a black Dodge Grand Caravan. Inside the temple gates, construction workers poured caulking in the footpaths that encircle the 35,000-square-foot edifice. Sinclair, along with two Church officials who joined the interview in a conference call from Salt Lake City headquarters, talked up the temple’s many references to Indiana culture. A gold tulip-tree leaf graces its gates, an homage to the state tree. A spire topped by a 1,000-pound golden statue evokes the Soldiers’ and Sailors’ Monument. And, of course, the limestone exterior brings to mind the state’s vast quarries of the sedimentary rock. On that May morning, as construction neared conclusion, the neoclassical architecture spoke to the aspiring permanence of the place. “We’re building a structure to last 1,000 years,” Sinclair says, “through the eternities.”

On August 23, Mormons (members of the LDS Church) will flock here to attend the official dedication of the temple, one of only 147 such structures in existence. Indiana Mormons (roughly 40,000 of them) who have been traveling to out-of-state temples on a monthly basis will finally have a house of worship to call their own.

If you don’t know much about the faith, you’re not alone. For as long as Mormons have been around, they’ve been dogged by questions about even their most basic doctrinal tenets and religious ceremonies. Do they really have secret handshakes? (Yes.) Do they practice polygamy? (No, only a fringe group of fundamentalist Mormons still do.) Do they really wear magical underwear? (It’s not magical, but the special white undergarments worn by Mormons in good standing serve as a daily reminder of their obligations under the covenant.) So in recent decades, Mormons have made a concerted effort to become more mainstream. On their website, they feature frequently asked questions ranging from topics such as the Heavenly Father to homosexuality. They’ve mounted sleek ad campaigns to bolster their believers. And where they once used flip charts to spread the good word, they now use iPads.

None of which, apparently, has improved their reputation for being secretive or stemmed the tide of defectors from the Church. According to several recent reports, membership is in decline nationwide, and the actual number of practicing Mormons may be a fraction of figures cited by the Church. All of which begs the question: Why build a colossal Mormon temple in Indiana, a state that has been little more than an afterthought in the faith’s 185 years of existence? The answer, as it turns out, is as shrouded in mystery as the ceremonies that will soon take place inside the temple’s walls.

In 1831, two Mormon missionaries first set foot in Indiana. That year, they proselytized in Madison, Unionville, and Vienna. They arrived here only a year into the existence of their belief system, derived from four key religious texts: the Bible, the Pearl of Great Price, the Doctrine and Covenants, and the Book of Mormon, the last of which the prophet Joseph Smith claims to have found buried in a hill near Manchester, New York, after being tipped off to its location by the angel Moroni.

Broadly, Smith’s revelations suggested that sometime after the death of Jesus Christ, a widespread period of apostasy ensued. Christians drifted. And Smith believed it was his calling to return the church to the one the Heavenly Father had originally envisioned, including building the kind of sacred temples mentioned in the Old Testament. After Jesus’s death, Smith maintained, He had appeared to his followers in North America, so it was a sacred place, too.

In 1832, Smith himself visited Greenville, Indiana. The Church’s early congregations in Ohio and Missouri dwarfed those of Indiana. In time, though, “congregations in Indiana sprouted up along the travel routes between the two states,” says Keith Erekson, director of the LDS Church History Library in Salt Lake City. By 1843, there were branches of the Mormon Church in 30 Hoosier counties. A year later, a mob of more than 100 men attacked a prison in Carthage, Illinois, where Smith had been held for allegedly inciting a riot after shuttering the Nauvoo Expositor, a local newspaper opposed to his teachings. The mob killed Smith and his brother, Hyrum. Brigham Young took the reigns of Smith’s church, leading his people westward to Utah, a place they called “Zion”—God’s country.

Meanwhile, back in Indiana, Mormons gathered in homes and rented halls for casual discussions of their faith. Counting just 10 families among its Indianapolis Church rolls in 1913, the Saints built their first meetinghouse here for weekly services in 1927. But to perform temple ceremonies such as being “sealed” in marriage and the “baptism of the dead,” the faithful had to travel to Logan, Utah, where their brethren had fled to escape persecution for their beliefs.

More meetinghouses in Indiana followed. After World War II, Mormon veterans, spurred by the G.I. Bill, began to pour into the state’s colleges and universities. In 1959, Saints established the state’s first “stake”—a group of local congregations known in the Church as “wards.” Over the next five decades, 10 additional stakes appeared across the state. By 2010, the number of Saints had reached 41,290.

Today, Indiana’s Mormons are a blend of transplants and native Hoosiers. In recent years, Central Indiana’s wards have been home to a rotating cast of prominent Mormons, from high-profile attorneys such as Sinclair to media personalities such as former WISH-TV anchor Rick Hightower, from former Colts wide receiver Austin Collie to former Pacers forward Mark Pope. On Sundays, Mormons convene for three-hour meetings, sharing testimonies and singing hymns. After an hour-long worship session, the men and women split, meeting separately for gender-specific instruction in righteous living. On Wednesday nights at their picturesque brick White River Chapel, young Mormons convene at the Church’s local singles meetup, hoping to find an eternal mate. Mormons don’t believe that marriages end at “death do us part,” and their marriage ceremonies seal couples for all eternity.

But for those most sacred ceremonies, Indiana’s Mormons have long been required to travel to temples in Chicago; Louisville; Columbus, Ohio; and even Washington, D.C. “Members in Indiana prayed for years that a temple would be built in this state,” Sinclair says. Those prayers were answered in October 2010, when Church president Thomas Monson announced his latest celestial revelation, and local Saints learned that the road to heaven would soon take them through the roundabouts of Carmel.

For the Mormon church nationwide, building its people has proven difficult recently. Three years ago, Elder Marlin Jensen, the Church’s then-outgoing historian, admitted that its Saints were leaving in droves, according to a special report by Reuters, citing evidence that the ultraconservative Book of Mormon was a contributing factor to the exodus. Worldwide, the Church claims to number 15 million, but some estimates peg the figure of practicing Mormons at just a fraction of that, around 5 million. An April report in the Deseret News, a Salt Lake City newspaper owned by the LDS, noted declining conversion rates achieved by its missionaries: 3.4 converts per missionary per year compared with an average of 5 in the last decade. In an email exchange with IM, Sinclair denied that those reports reflect the trend here. Instead, he says, Mormonism is actually growing in Indiana, counting 400 baptisms statewide last year. But it has been doing so at a much slower rate than in years past. From 1990 to 2000, Church rolls increased by more than 50 percent here. The following decade, that slowed to 22 percent. And over the last five years, the Church’s growth trundled to 6 percent.

Sinclair admits that “there will always be some who choose to end their association with the Church.” One of those members shared her personal story at two evangelical Christian churches on Indy’s south side a few months ago. Lynn Wilder—a former Indiana special-education teacher, a leader in the Indianapolis North Stake, and an ex–Brigham Young University tenured professor—left Mormonism in 2008. In a two-and-a-half-hour appearance, Wilder shared a 91-slide presentation called “Unveiling Grace: The Story of How We Found Our Way Out of the Mormon Church and Into the Arms of Grace.” (She authored a book with a similar title in 2013.) In part, Wilder said, the Church’s secrecy led her and her family away from it. She described some of the temple’s rituals that are most shrouded in mystery. While marriage “sealings” are one of the major ordinances that take place inside of temples, baptisms of the dead are probably the most frequent. Mormons spend countless hours tracing their genealogies in the hopes of baptizing long-deceased ancestors. By doing so, they believe they give their dead relatives a chance at salvation. Wilder traced her own genealogy back through the 16th century and spent hours over the course of several years listing her ancestors’ names in a baptism of the dead at the Chicago temple.

In American culture, parody and suspicion have filled the vacuum created by the Church’s tight-lipped approach to its practices. The faith has been satirized by the hit musical Book of Mormon—“a little too salty for my taste,” Sinclair admits. And it has been bandied about on cable television in shows such as HBO’s Big Love.

Sinclair and other Saints have taken the spotlight with relative good humor: The Church purchased ad space in Book of Mormon playbills, running taglines such as “The book is always better.” As for the Indiana temple, Sinclair downplays the notion that the Church’s decision to open it here had anything to do with the number of Mormons in the area going up or down. “It is not a reward,” Sinclair says. “I view it as an invitation to build our community.”

At the church’s General Conference in October 2010, president Thomas Monson announced five new temples. The Church would build in Lisbon, Portugal; Hartford, Connecticut; Urdaneta, Philippines; Tijuana, Mexico; and Carmel, Indiana. How exactly the Church settled on this area remains between Monson and the Heavenly Father, says Sinclair, adding that “decisions about where and when to build temples are made by the president.”

At the Church’s General Conference in April, Elder Neil L. Andersen of the Quorum of the Twelve Apostles insisted temple locations were divine decisions, not demographic ones. “The location of a temple is not a convenient geographical decision,” Andersen said. “It comes by revelation from the Lord to His prophet, signifying a great work to be done.” Sinclair declined to answer other questions about the local temple project, from how much it cost to how a construction crew was vetted for so sensitive an undertaking. But in January 2011, the Church’s real-estate office in Salt Lake City announced it had chosen a 50-acre plot on 116th Street. In a ceremony that included Church dignitaries standing alongside Carmel Mayor Jim Brainard, the Church broke ground in September 2012. Construction crews completed the monumental building early this summer.

Inside, 12 oxen statues now hold up a large baptismal font. A “celestial room,” clad in white and bedecked with a dazzling chandelier, invites Saints to reflect in an environment that evokes heaven. After three weeks of tours that began in July and end August 8, the public won’t be allowed to see any of it ever again (though there’s a “Visitors Welcome” sign on the meetinghouse building nearby). Temple ceremonies will begin on the day of the dedication; local faithful will start worshipping regularly in the chapel in September.

As much as the Church has attempted to fight the strong headwinds of its current cultural moment, it has no intention of becoming more transparent. That could further Mormonism’s declining growth. The Church’s message won’t “change in response to public opinion, and it is not for any particular demographic or generation—it is eternal,” Sinclair says. When you drive by the new temple, you can marvel at its shining statue of Moroni and its gleaming limestone facade. But don’t count on unraveling the mysteries of how it arrived here or what happens inside. Those belong only to the Mormons.