…To reach people we must appreciate and adapt to their culture, but we must also challenge and confront it. This is based on the biblical teaching that all cultures have God’s grace and natural revelation in them, yet they are also in rebellious idolatry. If we overadapt to a culture, we have accepted the culture’s idols. If, however, we underadapt to a culture, we may have turned our own culture into an idol, an absolute. If we overadapt to a culture, we aren’t able to change people because we are not calling them to change. If we underadapt to a culture, no one will be changed because no one will listen to us; we will be confusing, offensive, or simply unpersuasive. To the degree a ministry is overadapted or underadapted to a culture; it loses life-changing power.
More GREAT Keller quotes here… in fact… we website that is devoted to them!

As usual, Keller hits the nail on the head.
What gets real confusing is when the community you are called to reach is filled with church folk or nice moral people who don’t yet understand Kingdom style living. In that case their culture IS your culture; it is the motives and perspective which differ.
Of course Keller also covers that situation in detail in his Prodigal God materials.