SALT LAKE CITY — For Heather Gay, becoming a cast member on “The Real Housewives of Salt Lake City” was like “drawing a line in the sand” with her identity as a member of the Church of Jesus Christ of Latter-day Saints. When she joined the Bravo show in 2019, she realized her move to quietly quit the faith would become very loud.

In fact, she knew many things she had left unspoken for years would finally be said.

For three seasons, fans of the reality show (and there are many) have watched the bubbly, witty Ms. Gay, who was divorced when she joined the show, flirt with men, party and criticize her former religion — breaking the rules of it openly.

But “The Real Housewives of Salt Lake City” is responsible for some of the most dramatic story lines in the franchise — one cast member pleaded guilty to widespread fraud in federal court and another is married to her step-grandfather — and Ms. Gay’s struggles with her faith are often relegated to B or even C plots. “Bad Mormon,” Ms. Gay’s new memoir, which comes out on Feb. 7, is her opportunity to explore her own feelings at length.

Ms. Gay, 48, details the pain, confusion, shame and depression she experienced during four-and-a-half decades of loyal membership in the church, and her eventual decision to leave. She describes being asked questions about her sexual experiences by her bishop when she was a teenager (a practice others have spoken out about); “preaching eternal life while dying inside” as a 20-something missionary in the south of France; and pushing through a decade of a zombielike existence with her ex-husband.

In June 2022, Ms. Gay filed a trademark application on the words “Bad Mormon” for use on merchandise like beer mugs, shirts and socks. The church is opposing the application, claiming that Ms. Gay’s trademark intends “to cause confusion, tarnishment and dilution,” and that her use of the word “bad” is “deceptive in that it falsely represents that Opposer, the Church or its members are bad or otherwise behaving immorally or contrary to the teachings of the Church.” The case is currently scheduled for oral arguments in May 2024. The church did not offer any comment on the matter.

“When the book comes out, there will be no path back,” Ms. Gay said. She was seated on a swivel chair in front of her laptop in a small office at Beauty Lab + Laser, the medical spa business she founded in Salt Lake City with her close friend Dre Robinson in 2017. “I don’t know what the fallout will be. But I’m not scared like I used to be,” she said.

For years, Ms. Gay said she worked fastidiously to maintain the facade of a perfect life.

“She was beautiful and wealthy and married to this guy with these beautiful kids,” said Ms. Robinson, 42, who met Ms. Gay in 2008 when their daughters were in kindergarten together, adding that she always called her “the queen of the moms.”

But then, in 2011, Ms. Gay’s husband left, and the veneer she had constructed crumbled. They divorced a few years later, a big no-no for members of the church. She started dating different men, building a business and drinking alcohol. In 2019, she stepped away from the church.

And yet, Ms. Gay said, “I didn’t get hit by lightning, and I’m happier than I’ve ever been.” She said she’s currently in the process of having her name expunged from membership records, the final piece of her exit.

“I loved God, my friends, my family, that we all believed the same exact things,” she said. “It was very safe. It was family endlessly. It informed every choice I ever made. I believed it was the only way to be happy and to be saved.”

Ms. Gay, the third of six siblings, was born in Carmel-by-the-Sea, Calif., and moved to Denver with her family when she was 5. (Ms. Gay and her siblings were raised in the church, and most of her family are still fully involved in it.)

As a teenager, Ms. Gay walked the line, participating in church activities and taking its teachings to heart while surreptitiously trying alcohol, kissing boys and watching forbidden TV shows and films like “Big Bad Mama” when her parents were out, she writes in the book.

At 21, she graduated from Brigham Young University with a degree in Humanities but saw herself as a failure because she hadn’t found a husband. Unsure of what to do next, she decided to travel abroad to do missionary work.

After returning to the United States, she married Mr. Gay and gave birth to three daughters, Ashley, Georgia and Annabelle. Cracks in her marriage appeared as early as the honeymoon, she writes, but Ms. Gay refused to give up.

Eventually, her husband left. “The divorce was the dam that broke,” she said. Ms. Gay felt isolated. Her stress levels were very high, she said, and she developed Bell’s palsy, with half of her face becoming temporarily paralyzed. In 2016, single and struggling with her religion, Ms. Gay began working on a medical spa business with Ms. Robinson, who had left the church many years ago, and became her “non-Mormon guide.”

“I could have really candid conversations with her,” Ms. Robinson said. “I think it was scary for her.”

Slowly, Ms. Gay began to distance herself from the church, driven, she said, by the divorce, by the church’s treatment of L.G.B.T.Q. people and women, and by her desire for her daughters to live and love freely, to not define themselves by their purity and ability to stay married and have children.

Ms. Gay said that writing her memoir was the first time she really gave herself space to understand her story and her experiences.

Right now, she is single, and she isn’t seriously looking for a partner.

“I’m trying to figure out who I am outside of all of this,” she said. Her eyes filled with tears. “I don’t know that I’ve ever truly been in love because I’ve never truly been who I was meant to be. And it makes me sad. I would love to fall in love and have a partner, but I don’t even know how much therapy that’s going to take.”

Though Ms. Gay is considered a fan favorite, viewers have taken issue with her support of Jen Shah, another cast member, who pleaded guilty to conspiracy to commit wire fraud in federal court in July, andwas sentenced to six and a half years in prison last month.

“The timing and circumstance of it made me especially susceptible to blindly supporting her,” she said over lunch at Beaumont Bakery and Cafe, minutes away from where she grew up.

The cafe is a spot she frequents, and it is just a 10-minute drive from her current house, the house where she lived with her ex-husband, and the house where she spent her teenage years with her parents.

She supported Ms. Shah for so long, she said, because “I believe in redemption.” Ms. Gay said that she also felt responsible for Ms. Shah because she was the one who connected Ms. Shah with the show’s casting directors.

“I felt like I brought her into this and I felt like it exacerbated a lot of her circumstances,” Ms. Gay said.

When Ms. Gay first joined the cast, she reveled in the opportunity to shine a light on herself and on her now-former religion, to stop performing goodness and to finally let herself be a little bit bad. Now, she recognizes the platform the show has given her to reach others who are doubting themselves and feel isolated.

The show has encouraged her to live to the fullest extent, she said, and take the opportunities she gets from being a public figure.

“I don’t need to be the fan favorite,” she said. “I just need a seat at the table for a few more years, to get what I can get, you know? To suck the marrow from the bone. I feel like I’m living on borrowed time. I owe it to myself, to suck up these experiences, good, bad and ugly and just to kind of live foolhardy.”