I ordinarily begin speaking about sin to a young, urban, non-Christian like this:
Sin isn’t only doing bad things, it is more fundamentally making good things into ultimate things. Sin is building your life and meaning on anything, even a very good thing, more than on God. Whatever we build our life on will drive us and enslave us. Sin is primarily idolatry.
Why is this a good path to take?
First, this definition of sin includes a group of people that postmodern people are acutely aware of. Postmodern people rightly believe that much harm has been done by self-righteous religious people. If we say “sin is breaking God’s law” without a great deal of further explanation, it appears that the Pharisaical people they have known are ‘in’ and most other people are ‘out.’ Pharisees, of course, are quite fastidious in their keeping of the moral law, and therefore (to the hearer) they seem to be the very essence of what a Christian should be. An emphasis on idolatry avoids this problem. As Luther points out, Pharisees, while not bowing to literal idols, were looking to themselves and their moral goodness for their justification, and therefore they were actually breaking the first commandment. Their morality was self-justifying motivation and therefore spiritually pathological. At the bottom of all their law-keeping they were actually breaking the most fundamental law of all. When we give definitions and descriptions of sin to postmodern people, we must do so in a way that not only challenges prostitutes to change but also Pharisees.
There is another reason we need a different definition of sin for postmodern people. They are relativists, and the moment you say, “Sin is breaking God’s moral standards,” they will retort, “Well, who is to say whose moral standards are right? Everyone has different ones! What makes Christians think that theirs are the only right set of moral standards?” The usual way to respond to this is to become sidetracked from your presentation of sin and grace into an apologetic discussion about relativism. Of course, postmodern people must be strongly challenged about their mushy view of truth, but I think there is a way to move forward and actually make a credible and convicting gospel presentation before you get into the apologetic issues. I do it this way, I take a page from Kierkegaard’s The Sickness Unto Death and I define sin as building your identity—your self-worth and happiness—on anything other than God. Instead of telling them they are sinning because they are sleeping with their girlfriends or boyfriends, I tell them that they are sinning because they are looking to their careers and romances to save them, to give them everything that they should be looking for in God. This idolatry leads to drivenness, addictions, severe anxiety, obsessiveness, envy of others, and resentment.
I have found that when you describe their lives in terms of idolatry, postmodern people do not offer much resistance. They doubt there is any real alternative, but they admit sheepishly that this is what they are doing. I have also found that this makes sin more personal. Making an idol out of something means giving it the love you should be giving your Creator and Sustainer. To depict sin as not only a violation of law but also of love is more compelling. Of course a complete description of sin and grace includes recognition of our rebellion against God’s authority. But I’ve found that if people become convicted about their sin as idolatry and mis-directed love, it is easier to show them that one of the effects of sin is to put them into denial about their hostility to God. In some ways, idolatry is like addiction writ large. We are ensnared by our spiritual idols just like people are ensnared by drink and drugs. We live in denial of how much we are rebelling against God’s rule just like addicts live in denial of how much they are trampling on their families and loved ones.
Keller, Tim (2008) Talking About Idolatry in a Postmodern Age