Is the doctor in the town of Megaton. He'll patch you up or heal your radiation for 100 caps, like the other doctors across the wasteland. He seems a bit moody, and he has a negative reaction to the player, saying things like "You better be near dead if your coming in here".
If you pass a speech check Churcch tells you Leo Stahl has an addiction to jet, and you can find him at the Water Purification Plant at night getting high. This piece of information can help you clear The Power of Atom, because if your explosive skill isn't high enough Leo can sell you mentants.
There is a holotape you can take on Church's desk to gain more information or start The Replicated Man.