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.