The Preachers are pedestrians in GTA IV that shout about the doom of humanity. One is located across the street from the South Bohan Safehouse and another is located standing in front of the chapel building in the Alderney State Correctional Facility prison yard. Many others can be found near churches, banks, police stations, and other random locations around Liberty City.
They appear to be rather eccentric and delusional. All of the stuff they preach about is definitely false. Even so, they believe that what they are saying is true. They are most likely a play on the "bible belt" fundamentalist preachers who often come into large cities (such as Washington, D.C. and New York City) to preach "the good word" to anyone passing by.
There are two different character models of preachers each with there own view on what hell is in store for those who are nonbelievers and the "heaven" that those who believe will go to when they die. If you listen to them, you can hear them say some pretty odd and interesting things.