"The decision to leave is you're giving up everything you've worked for your whole life," she says in the clip. "I feel that people need to understand this has been my whole life and I want them to understand how it happens. And so as time goes on, you start to lose touch with the real world."
Remini describes during the interview how she and her family joined Scientology when she was a child and continued with her commitment to the Church for 30 years until she parted ways as an adult, according to ABC. "The mindset becomes us against them," she explains in the clip.
"Being critical of Tom Cruise is being critical of of Scientology itself ... you are evil."
The hour-long special airs Oct. 30 at 7 p.m. on ABC.
Leah Remini to discuss Scientology on '20/20': 'Being critical of Tom Cruise is being critical of Scientology itself'
BEVERLY HILLS, CA - JANUARY 14: Actress Leah Remini arrives to the 2014 UNICEF Ball Presented by Baccarat at the Regent Beverly Wilshire Hotel on January 14, 2014 in Beverly Hills, California. (Photo by Alberto E. Rodriguez/Getty Images)
DANCING WITH THE STARS - 'Episode 1710' - Leah Remini and Tony Dovolani were the next couple to be eliminated on 'Dancing with the Stars,' MONDAY, NOVEMBER 18 (8:00-10:01 pm, ET), on the ABC Television Network. (Photo by Adam Taylor/ABC via Getty Images)
TONY DOVOLANI, LEAH REMINI, BROOKE BURKE-CHARVET, TOM BERGERON
Leah Remini arrives at the 15th Annual DesignCare on Saturday, July 27, 2013 in Malibu, Calif. (Photo by Richard Shotwell/Invision/AP)
Leah Remini attends the 3rd Annual My Brother Charlie Family Fun Festival at Culver Studios on Sunday, Aug. 12, 2012 in Culver City, Calif. (Photo by Todd Williamson/Invision/AP)
Leah Remini arrives at the Disney ABC Television Group 2012 Summer Press Tour All-Star Cocktail Reception on Friday, July 27, 2012, in Beverly Hills, Calif. (Photo by Matt Sayles/Invision/AP)
FILE - In this April 22, 2010 file photo, actress Leah Remini arrives at the Us Weekly Hot Hollywood Style Celebration in Los Angeles. (AP Photo/Matt Sayles, file)
Actress Leah Remini arrives at the CBS CW Showtime press tour party in Beverly Hills, Calif. on Wednesday, July 28, 2010. (AP Photo/Dan Steinberg)
Actress LEAH REMINI at the Los Angeles premiere of Swordfish. 04JUN2001. Paul Smith/Featureflash
los angeles sep 16 leah...
LOS ANGELES, CA - FEBRUARY 03: A view of the Church of Scientology International in Hollywood on February 03, 2014 in Los Angeles, California. (Photo by Axelle/Bauer-Griffin/GC Images)
The Hollywood Church of Scientology Celebrity Centre.
Church of Scientology, Toronto Head Office
celebrity scientology church los angeles la
CHURCH OF SCIENTOLOGY EUROPEAN OFFICE FOR PUBLIC AFFAIRS AND HUMAN RIGHTS 91 RUE DE LA LOI BRUSSELS