Mormon leaders have acknowledged for the first time that the church’s founder and prophet, Joseph Smith, portrayed in church materials as a loyal partner to his loving spouse Emma, took as many as 40 wives, some already married and one only 14 years old.

The church’s disclosures, in a series of essays online, are part of an effort to be transparent about its history at a time when church members are increasingly encountering disturbing claims about the faith on the Internet.

Many Mormons, especially those with polygamous ancestors, say they were well aware that Smith’s successor, Brigham Young, practiced polygamy when he led the flock in Salt Lake City. But they did not know the full truth about Smith. “Joseph Smith was presented to me as a practically perfect prophet, and this is true for a lot of people,” said Emily Jensen, a blogger and editor in Farmington, Utah, who often writes about Mormon issues.

She said the reaction of some Mormons to the church’s disclosures resembled the five stages of grief in which the first stage is denial, and the second is anger. Members are saying on blogs and social media, “This is not the church I grew up with, this is not the Joseph Smith I love,” Ms. Jensen said.

Smith probably did not have sexual relations with all of his wives, because some were “sealed” to him only for the next life, according to the essays posted by the church. But for his first wife, Emma, polygamy was “an excruciating ordeal.”

The four treatises on polygamy reflect a new resolve by a church long accused of secrecy to respond with openness to the kind of thorny historical and theological issues that are causing some to become disillusioned or even to abandon the faith.

The Church of Jesus Christ of Latter-day Saints, as the Mormon Church is formally known, has quietly posted 12 essays on its website over the last year on contentious topics such as the ban on blacks in the priesthood, which was lifted in 1978, and accounts of how Smith translated the Book of Mormon, the church’s sacred scripture.

Elder Steven E. Snow, the church historian and a member of its senior leadership, said in an interview, “There is so much out there on the Internet that we felt we owed our members a safe place where they could go to get reliable, faith-promoting information that was true about some of these more difficult aspects of our history. “We need to be truthful, and we need to understand our history,” Elder Snow said. “I believe our history is full of stories of faith and devotion and sacrifice, but these people weren’t perfect.”

The essay on “plural marriage” in the early days of the Mormon movement in Ohio and Illinois says polygamy was commanded by God, revealed to Smith and accepted by him and his followers only very reluctantly. Abraham and other Old Testament patriarchs had multiple wives, and Smith preached that his church was the “restoration” of the early, true Christian church.

Most of Smith’s wives were between the ages of 20 and 40, the essay says, but he married Helen Mar Kimball, a daughter of two close friends, “several months before her 15th birthday.” A footnote says that according to “careful estimates,” Smith had 30 to 40 wives. The biggest bombshell for some in the essays is that Smith married women who were already married, some to men who were Smith’s friends and followers.

The essays held nothing back, said Richard L. Bushman, emeritus professor of history at Columbia University and author of the book “Joseph Smith: Rough Stone Rolling.” Dr. Bushman said of church leaders: “Somewhere along the line they decided they were just going to tell the whole story, not to be defensive, not to try to hide anything.

And there’s no single fact that’s more unsettling than Joseph Smith’s marriage to other men’s wives. “It’s a recognition of maturity,” said Dr. Bushman, who is a Mormon. “There are lots of church leaders who say: ‘We can take anything, just let us know how it really happened. We’re a church that is secure.’ ” The younger generation of Mormons will benefit from this step, said Samantha Shelley, co-founder of the website MillennialMormons.com in Provo, Utah.

She said she knew of Smith’s polygamous past, but “it’s so easy for people these days to stumble upon something on the Internet, and it rocks their world and they don’t know where to turn.” In 1890, under pressure by the American government, the church issued a manifesto formally ending polygamy. The church’s essay on this phase admits that some members and even leaders did not abandon the practice for years.

But the church did renounce polygamy, and Mormons who refused to do the same eventually broke away and formed splinter churches, some that still exist. Warren Jeffs, the leader of one such group, was convicted in Texas in 2011 of child sexual assault.

There remains one way in which polygamy is still a part of Mormon belief: The church teaches that a man who was “sealed” in marriage to his wife in a temple ritual, then loses his wife to death or divorce, can be sealed to a second wife and would be married to both wives in the afterlife.

However, women who have been divorced or widowed cannot be sealed to more than one man. Kristine Haglund, the editor of Dialogue: A Journal of Mormon Thought, said that while she found the church’s new transparency “really hopeful,” she and other women she had talked with were disturbed that the essays do not address the painful teaching about polygamy in eternity.

“These are real issues for Mormon women,” Ms. Haglund said. “And because the church has never said definitively that polygamy won’t be practiced in heaven, even very devout and quite conservative women are really troubled by it.”

The church historian, Elder Snow, said that the process of writing the essays began in May 2012. Each one was drafted by a scholar, often outside the church history department, then edited by church historians and leaders, and vetted by the church’s top authorities.

They may issue one more essay, on women and the priesthood, an issue that has grown increasingly controversial as some Mormon women have mobilized to challenge the male-only priesthood. The church has not publicly announced the posting of the essays, and many Mormons said in interviews that they were not even aware of them. They are not visible on the church’s home page; finding them requires a search or a link.

Elder Snow said he anticipated that the contents would eventually be “woven into future curriculum” for adults and youths. The church recently released an informational video about the distinctive Mormon underwear called “temple garments” — and it received far more attention among Mormons and in the news media than the essays on polygamy.

Sarah Barringer Gordon, a professor of constitutional law and history at the University of Pennsylvania, and a non-Mormon who has studied the Mormon Church, said it had dealt with transparency about its past before this, addressing Mormon leaders’ complicity in an attack on a wagon train crossing southern Utah in 1857, known as the Mountain Meadows massacre.

But she said this recent emphasis on transparency by the church was both unprecedented and smart. “What you want to do is get out ahead of the problem, and not have someone say, ‘Look at this damaging thing I found that you were trying to keep secret,’ ” she said.