It's hard to comprehend as a Christian sometimes how the earliest Christians after initial Jewish converts were more likely from urbanized and postmodern areas rather than rural areas. Early Christianity grew in these regions yet suffered persecution and rejection mainly among rural and military groups. Christianity continues to be a more multi-cultural and universalizing religion, something Pastor Keller rightfully points out. And much of the Christian mission fields will be in urbanized areas as the world starts to leave rural areas for cities and suburban areas get annexed into metropolitan areas. The city is definitely a great mission field but it'll be very interesting as many of those who profess Christianity are also likely to be pluralistic and/or only nominally Christian as well.