"As time goes on, you start to lose touch with the real world. The mindset becomes 'us against them,'" Remini tells "20/20" of her time in the church, which she joined along with her family as a young girl. "The decision to leave is you are giving up everything you have ever known and everything you have worked for your whole life."
Regarding Cruise, Remini says that criticism of the actor is not tolerated within Scientology. "Being critical of Tom Cruise is being critical of Scientology itself. You are evil," she says.
Watch a sneak peek of Remini's "20/20" interview here:
Remini's memoir, "Troublemaker: Surviving Hollywood and Scientology," will be released on Nov. 3. Her 20/20 interview airs Friday at 10/9c on ABC.