Park's relationship with Choi and her father Choi Tae-min has long been controversial in Korea.
Choi named himself a modern day Buddha and called for all people to strive for eternal life.
What influence did Choi Soon-sil have?
After her father died in 1994 at age 82, Choi Soon-sil succeeded him as church leader and spiritual mentor to Park, as the former first daughter became a political force of her own.
"It's much more than simply, 'oh she knows this person,' it's deeply intertwined, almost like they're Rasputin and Park Geun-hye is just a puppet."
Though she never held an official position, recent revelations showed that Choi was given advance access to presidential speeches and other documents.
Local media and opposition parties have accused Choi of abusing her relationship with the president to force companies to donate millions of dollars to foundations she runs.
In a televised apology, Park said Choi looked at "some documents" for a certain period of time after Park took office, but didn't specify what they were.
"I am shocked and my heart is breaking for causing public concern," Park said.
Shamanism and cults
While much of the outrage has surrounded Choi's alleged influence over Korean politics, the scandal has also shone a light on the prevalence of cults and alternative religious movements in the country.
Korean shamanism, or Muism, is an ethnic Korean religion with similarities to Japanese Shintoism.
While Christianity and Buddhism dominate Korean religious life, some churches incorporate shamanistic practices and many people consult with fortune tellers or shamans.
Stemming the scandal
South Korean presidents are immune from prosecution for anything but insurrection or treason, and Park's term wasn't due to end until 2018.
Prime Minister Hwang Kyo-ahn is now acting President, and will hold the post for the duration of the court's deliberation.
Reputational fallout
Park's approval ratings have plummeted, hampering her administration's ability to tackle key economic and security issues.
The scandal exacerbated her Saenuri Party's weak standing in the National Assembly following April elections.
North Korea was also quick to rain criticism on Park, a longtime target of the country's propaganda machine.
"No one has ever heard of such an ugly political scandal involving the President memorizing speeches inspected by a random swindler anywhere else in the world."
CNN's Euan McKirdy, Elizabeth Joseph and Yoonjung Seo contributed reporting.