The church showed "complete, even cruel, indifference to those having suffered abuse", a new report has found.
Shincheonji Church of Jesus is led by a man, Lee Man-hee, claiming to be the 'promised pastor' mentioned in the bible.
"We should all be mindful of how we came to believe the things we do," lead author Dr Thomas Swan said.
They had been "shut up", forbidden to communicate with anyone.
The private investigators have watched ex-Brethren from cars and parked surveillance vans.
"How much better our world would be without these instruments of death!"
Many churches opened their doors to people of all faiths.
"It has confused and saddened a lot of people."
"Their interpretation of the scripture will not allow them to accept gay people."