David Miscavige: Five Things to Know About Scientology's Controversial Leader

David Miscavige has been one of the most prominent figures in Scientology for more than 30 years – and one of the most controversial.

Now, the Scientology leader, 55, is under renewed scrutiny as his own father, Ronald Miscavige, prepares to publish a memoir about his estranged son, titled Ruthless: Scientology, My Son David Miscavige, and Me.

But just who is David Miscavige? Here are five things you need to know:

1. David's father introduced him to Scientology as a child, and he rose quickly within the church

Ronald Miscavige introduced young David and his family to Scientology in the late-'60s, and said the boy who struggled with severe asthma and often got into fistfights connected with the religion immediately.

"After 45 minutes, David walks out, smiling, bright," Ronald tells 20/20 in an upcoming interview about the book. "[That moment] decided his life, and mine."

Earning a reputation as a Scientology prodigy, David was reportedly conducting auditing sessions – a kind of Scientology counseling session – by age 12. At 16 he quit high school to join "Sea Org," a group featuring Scientology's most dedicated members.

"He advanced very rapidly in the Church," Ron Miscavige tells 20/20 "He really decided at a very young age to make it his career and his mission."

2. He was close to Scientology founder L. Ron Hubbard – but clashed with other members of Hubbard's family

Miscavige began working directly with Scientology founder L. Ron Hubbard when he was just a teenager, at 17 becoming director of photography for Scientology training films Hubbard was producing in LaQuinta, California. According to the St. Petersburg Times, Miscavige rose to action chief of the Commodore's Messenger Organization, a group that investigated claims against the church, by age 19.

Within that capacity, a 21-year-old Miscavige stepped forward in 1981 to boldly oust Hubbard's wife, Mary Sue Hubbard, from her high-ranking role in the church's Guardian's Office after she and other church officials were convicted of conspiring to steal federal government documents and cover it up. (Hubbard's estranged son Ronald DeWolf also sued Miscavige in the early '80s, accusing him of embezzling and manipulating his father. The case was dismissed in 1983.)

"People keep saying, 'How'd you get power?' " Miscavige told the St. Petersburg Times in a rare interview in 1998, regarding his ouster of Mary Sue Hubbard. "Nobody gives you power. I'll tell you what power is. Power in my estimation is if people will listen to you. That's it."

When L. Ron Hubbard died in 1986, it was Miscavige who delivered the news to Scientologists gathered at the Hollywood Palladium.

3. He is close friends with Tom Cruise and served as best man in his wedding to Katie Holmes

Miscavige is friendly with a number of celebrities who frequent Scientology's Celebrity Centre in Los Angeles, but has an especially close relationship with fellow Scientologist Tom Cruise, even serving as best man in Cruise's 2006 wedding to Katie Holmes. The celebration's star-studded guest list included Jennifer Lopez, Marc Anthony, Jim Carrey, Will Smith and Jada Pinkett Smith, Kirstie Alley, and Victoria Beckham, among others.

4. His wife, Shelly, has not been seen in public for years, which prompted Leah Remini to file a missing-person report

Although Miscavige was at Cruise's side during his 2006 wedding, there was one person notably missing from the nuptials: Miscavige's wife, Shelly. Actress Leah Remini, who released a memoir about her bumpy exit from Scientology last year, Troublemaker: Surviving Hollywood and Scientology, told 20/20 in an interview for the book that she was shocked Shelly was not at the wedding.

"Shelly was always where David Miscavige was," Remini said. "It was a wedding of the century ... it was like, 'Where’s Shelly?' "

In 2014, Remini filed a missing-person report that the LAPD deemed "unfounded" because they had spoken to Shelly. Last fall, Remini told PEOPLE she still doesn't know the whereabouts of her onetime friend.

"When I asked the officers, 'Did you see her with your own eyes? Is she alive and safe?' their response was, 'We cannot give you that information, ma'am,' " she says. "I still don't have an answer."

The church has long maintained that Shelly has never been missing and said in a statement to PEOPLE last fall, "Ms. Remini also continues her bizarre efforts to harass the Church of Scientology's leader and his wife, whom Ms. Remini has been obsessed with and has stalked for years."

5. Niece Jenna Miscavige Hill left the church, along with other relatives, and has become an outspoken critic

A number of Miscavige's relatives have left the church, but few have been so outspoken as his niece, Jenna Miscavige Hill. The daughter of David's older brother released a revealing memoir titled Beyond Belief in 2013, and spoke to PEOPLE last year before the release of the HBO documentary Going Clear: Scientology and the Prison of Belief.

"Even if something seems good at first, if an organization is telling you that you can't look at certain information or is keeping you away from your family, then it's time to reevaluate," Miscavige Hill said. "Maybe that's exactly what you should be doing."