July 29, 2013 12:00 PM

By all accounts, Leah Remini had the time of her life at the Italian wedding of her friend and fellow Scientologist Tom Cruise to Katie Holmes in November 2006. After the couple exchanged vows in a medieval castle, Remini, her husband, Angelo Pagan, and their pals Jennifer Lopez and then husband Marc Anthony returned to the luxurious Hotel Hassler for a nightcap at the cozy bar. There, the foursome laughed and talked into the wee hours. “She looked like she didn’t have a care in the world,” says an onlooker.

But the night did not end well. Puzzled that Shelly Miscavige, the wife of Scientology’s leader (and Cruise’s best man) David Miscavige, had not been at the wedding, Remini asked David where she was. According to multiple sources, Tommy Davis – then a church spokesman – sternly reprimanded her. The sources say Davis warned Remini not to ask about Shelly, who hasn’t been seen in public since 2007. (In 2011 Davis reportedly also dropped out of public view.) Church officials call the anecdote “ludicrous.” But after the wedding, Remini sources say she was required to undergo extensive “security checks” – an attitude-modification process involving intensive personal questioning. Then on July 12, news broke that Remini was leaving the controversial church after more than three decades. “I wish to share my sincere and heartfelt appreciation for the overwhelming positive response I have received from the media, my colleagues and from fans,” Remini, 43, said in a statement. Says her sister Nicole Remini-Wiskow: “Leah is saying, ‘I’m seeing things are wrong.’ She fights for what she believes is right.”

The church says it won’t comment on Remini or any parishioner in detail, and the actress has been silent since her statement. But the split comes as a shock to many. The brash Brooklyn-born star of The King of Queens (in which she played the outspoken Carrie Heffernan for nine seasons) had been one of Scientology’s most vocal Hollywood supporters. But “she is a tough woman and isn’t the type to yield or turn a blind eye to a blatant injustice,” says Jenna Miscavige Hill, the niece of David Miscavige and the author of Beyond Belief: My Secret Life Inside Scientology and My Harrowing Escape. “People like that don’t last in Scientology because they aren’t easy to control.”

Although as an infant Remini was baptized a Catholic, she became a Scientologist when she was 9 years old, along with her mom, Vicki Marshall, who eventually rose to one of Scientology’s highest ranks. Remini’s husband, restaurateur Angelo Pagan, with whom she has a daughter, Sofia, 9, has also taken Scientology courses. “Leah was very involved in the humanitarian things, the ministry efforts,” says Remini-Wiskow, 45, who left the church herself in 2005.

But after the incident at Cruise’s wedding, “Leah began questioning,” Remini-Wiskow says. She called the church to ask again about Shelly. “She said, ‘Can I call her? If [she’s] okay, I’ll move on.’ They said, ‘Write her a letter and we’ll give it to her.’ ” (The church spokesperson says Shelly Miscavige is a “private person” and denies she is being hidden from Remini or others.) Remini’s sister says Scientology officials started pulling Leah in and doing security checks at the church’s Clearwater, Fla., headquarters. Critics of Scientology say the practice is used to silence questioning members. “It’s an interrogation,” says Tony Ortega, a New York journalist who writes articles critical of the church. The church, her sister claims, also asked its members to “disconnect” from Remini and Scientologist relatives: “People she’s known for 30 years have completely stopped talking to her.” The church maintains that critics misunderstand the nature of security checks, denies that “disconnection” is used against members, and points out that Remini-Wiskow herself was not “disconnected” when she left the church.

Yet Remini-Wiskow says it was attempts to alienate the family that proved to be the last straw. “If they call my mother and say, ‘You have to choose between Leah and the church,’ how is that a church?” she says.

In the days since her decision became public, Remini, whose last prominent TV gig was a short-lived stint as a cohost of The Talk, “has been kind of holed up at home” in Los Angeles, says her sister. “None of her Scientology friends are sticking by her side. I guess you really learn who your friends are.”

Jennifer Lopez, a non-Scientologist, is one. “They’re best friends and they’re fine,” Nicole says. Remini hasn’t yet decided whether she’ll renounce all of Scientology’s teachings entirely. But whatever path she chooses, her family will be by her side. “My mom, my stepdad … everybody is supporting and backing her,” says Remini-Wiskow. “This isn’t going to break her—or us.”