The Heretic

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

Detailed walkthrough
Once you've joined the Chilren of Atom and spoken with Tektus, you'll want to report back to Richter. He'll ask you to hunt down 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 you some background on the missing Sister.

Afterwards, you're directed to the two holy sites to the east of The Nucleus - Glowing Grove and Radiant Crest Shrine. At each site, you'll find some torn pieces of a banner that Gwyneth used to make her graffiti messages. With all of the pieces, you find it's a banner advertising a campground called Waketak 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.

Next head east to the Ruined church, where you'll find Gwyneth praying. Initiate dialogue with her, at which point you 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.

Regardless of which option you choose, return to Grand Zealot Richter to complete the quest and receive Atom's Judgement - a unique super-sledge and one of the best melee weapons in the game.

Bugs
If you choose the option to spare Gwyneth and tell her to leave the island, she may still show up inside the Nucleus later doing random stuff like sweeping the floors and is completely unresponsive to any attempt to talk to her.