'People with a religious upbringing can grasp the idea of sin as the violation of God's moral law. That law can be explained in such a way that they realize they fall short of it. In that context, Christ and his salvation can be presented as the only hope of pardon for guilt. This, the traditional gospel of the last generation, is a "gospel for the circumcised."
However, Manhattan is also filled with postmodern listeners who consider all moral statements to be culturally relative and socially constructed. If you try to convict them of guilt for sexual lust, they will simply say, "You have you standards, and I have mine." If you respond with a diatribe on the dangers of relativism, your listeners will simply feel scolded and distanced. Of course, postmodern people must at some point be challenged about their mushy views of truth, but there is a way to make a credible and convicting gosple presentation to them even before you get into such apologetic issues.
I take a page from Kirkegaard's The Sickness Unto Death and define sin as building your identity - your self-worth and happiness - on anything other than God. That is, I use the biblical definition of sin as idolatry. That puts the emphasis not as much on "doing bad things" but on "making good things into ultimate things."
Instead of telling them they are sinning because they are sleeping with their girlfriends or boyfriends, I tell them they are sinning because they are looking to their romances to give their lives meaning, to justify and save them, to give them what they should be looking for from God. Their idolatry leads to anxiety, obsessiveness, envy, and resentment. I have found that when you describe their lives in terms of idolatry, postmodern people do not give much resistance. Then Christ and his salvation can be presented not (at this point) so much as their only hope for forgiveness, but as their only hope for freedom. This is my "gospel for the uncircumcised."'
Timothy Keller, 'The Gospel in All its Forms'. Available at: