Once in cities, Christians should be a dynamic counterculture. It is not enough for Christians to simply live as individuals in the city. They must life as a particular kind of community. Jesus told his disciples that they were "a city on a hill" that showed God's glory to the world (Matt. 5:14-16). Christians are called to be an alternative city within every earthly city, and alternate human culture within every human culture, to show how sex, money, and power can be used in nondestructive ways.
Tim Keller in "A New Kind of Urban Christian," Christianity Today, May 2006, p. 38.