The Heretic DLC, Far Harbor Quest
This quest will be triggered or can be aquired during What Atom Requires quest.
Related points of interest
Stages of "The Heretic" quest
1. Talk to Zealot Theil
Grand Zealot Richter told me to track down Sister Gwyneth, a former Child of Atom turned heretic. I should talk to Zealot Theil, who might know how to find her.