The Heretic

The Heretic is a Children of Atom quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
Once the Sole Survivor has joined the Children of Atom and spoken with Tektus, they will want to report back to Richter. He will ask them to hunt down Sister Gwyneth, a former sister who left the order and has been vandalizing the Children's altars around the island. First check with Zealot Theil, who attempted to find Gwyneth without success. She can give the player character some background on the missing sister.

Afterwards, the player character is directed to the two holy sites west of the Nucleus: Glowing Grove and Radiant crest shrine. At each site, there will be some torn pieces of a banner that Gwyneth used to make her graffiti messages. With all of the pieces, one finds that it is a banner advertising a campground called Kawaketak Station. The campground is an unmarked location, so head south and find it using the quest marker. Only one of the cabins can be entered, and on the floor to the left of the door is a holotape that completes the objective. A wolf pack will spawn outside upon setting foot into the cabin.

Next, head east to the ruined church, where one will find Gwyneth praying. Initiate dialogue with her, at which point the player character can attack her or learn more about her reasons behind the vandalism. A speech check will convince her to flee the island, while a more difficult speech check will convince her to return to the Children and beg forgiveness. Asking Gwyneth if she misses her family will drop the difficulty of the latter speech check.

Regardless of which option is chosen, return to Grand Zealot Richter to complete the quest and receive Atom's Judgement, a unique super sledge.

Bugs

 * If the option to spare Gwyneth and tell her to leave the Island is chosen, she may still show up inside the Nucleus later doing random actions like sweeping the floors and is completely unresponsive to any attempt to talk to her.
 * The holotape may load under the cabin and become unreachable. It can be dislodged using explosives.