The Preacher

 refers to an unmarked quest in Fallout 3.

There is a mad preacher ("Wastelander") standing on the second floor of a building in an alleyway west of the Reilly's Rangers headquarters entrance on Seward Square. The alley has been rigged with mines and nukes that he detonates if you try to get close to him. There is a hostage (another Wastelander) on one end of the alley, who you can talk to. If you are looking at the mad preacher when he detonates, he waves at you. He also dies in the blast.

Resolving the situation
If your stealth is high enough, you can sneak through and pick up the Frag Mines (at least five) and multiple Mini Nukes (there are at least five; one behind and one in between the barrels by the slide, one next to the shopping cart, one on some rocks across from the shopping cart, and one on the merry-go-round.) Even without the Light Step perk, the explosives will not detonate if you step on them, as they are intended to explode only when triggered by the preacher. Alternatively you can use Dogmeat to find ammo, to pick up the mininukes, or to find weapons, to pick up the Frag Mines.

If you remove all the nukes and mines off the ground, the Preacher can still cause detonation if he sees you in the alley. Removing all visible explosive doesn't stop him, there are at least five more Mini Nuke detonations. You can talk the hostage into confronting the man using Charisma, but he gets blown up in the mine field and you get negative karma. Only other solution appears to be killing the preacher (sniping or a Frag Grenade from below, for instance).

Reward

 * 5 Mini Nukes
 * 5 Frag Mines