This one is a side quest of the water purification series. Go to the Jefferson Memorial and talk to Scribe Bigsley about his problems with Megaton. *Note. You obviously won't have this quest if the town is destroyed.
Go to Megaton and figure out the source of the water theft. You can watch a woman named Rosa accidentally kill a beggar by giving him a bottle of Aqua Pura. She'll explain that it was supposed to be blessed water from a church in Springvale. You can also find out about the new cult through pamphlets from the beggars, a note on your door, or general exploration.
Go into Springvale to find the new church set up in an old house. Talk to Brother Gerard. He'll give you a bottle of holy water and demand that you drink it first. Drink it to realize that it is badly irradiated Aqua Pura. You need to get inside to figure this one out. Gerard won't let you into the church if you aren't irradiated. You have a few options here.
You can kill him and take the key. No one in the church notices. You can pick the lock on the trapdoor behind him. No one minds, but it is a very hard lock. You can also just irradiate yourself by going into the house behind him and flipping the switch by the bathtub. This turns on the radiation trap and lets you quickly irradiate yourself. Turn it off when you've got minor poisoning and go back. You can also just wear the ghoul mask if you completed the Tenpenny Tower quest already. Wearing this should let you convince him that you're actually a ghoul and are already quite irradiated.
Once you get into the Church, it's pretty simple. Talk to Mother Curie III and ask her about the holy water. She'll go into a religious speech where she admits that she's been stealing it from Atom's angels (The Brotherhood of Steel). You have three options to get her to stop.
You can just whip out a shotgun and kill her. You'll have to kill the other two worshippers in the church though. You'll also be attacked by the glowing one and the feral ghoul reaver who lurk in the basement. That's fairly tough in close quarters, so be ready for a fight before you do it. There isn't a karma loss for this.
You can pass a standard Speech check to convince her to stop. Only one shot at this though, so save first.
You can also irradiate yourself past 800 rads. This nearly lethal amount can be achieved by activating the radiation traps in the basement or the one outside. Irradiate yourself badly and then limp back to her. If you have more than 800 rads, she will see you as a messiah and accept your order.
As a side note, you can also technically finish this by blowing up Megaton in the Power of Atom quest. If you detonate the bomb, then Brother Gerard will be a ghoul outside the building. He'll peacefully wait by the wreckage to provide help for Atom's followers. Mother Curie III will become a Feral Ghoul Reaver. Bigsley will be shocked at the destruction but still give you your reward.
Go back and talk to Scribe Bigsley for your well earned caps.