In Meeting Jesus Again for the First Time, Marcus J. Borg points out a decisive difference between Jesus and the prevailing Jewish culture of his day. The prevailing Jewish culture was based on attaining holiness, which Borg refers to as a "purity system." The ministry of Jesus was based on the compassion of God, which is why Jesus associated with tax collectors, prostitutes, cripples, and other "impure" persons. Borg uses this difference to illustrate the political impact of Jesus's teachings, which contradicted much of the social and economic structure of his times.
This description of Jesus, and the contrast with prevailing Judaism, is consistent with Stephen Mitchell's The Gospel According to Jesus, but Mitchell makes a slightly different theological point. Mitchell agrees that Jesus's associations with "the wicked" were shocking to the Pharisees, but not for the reason we usually assume. The popular current view (or at least what I was taught in Sunday school) is that Jesus believed in forgiveness while the Pharisees did not. So Jesus was willing to associate with sinners because he wanted to redeem them and convert them, while the Pharisees did not want to redeem sinners and opposed Jesus's efforts. Mitchell says that this is nonsense.
Even the Pharisees believed in forgiveness and believed that God would forgive sinners. However, in order to be forgiven by God, a sinner must first repent. It may also be necessary to do a form of penance, or perform cleansing rituals, in order to be purified again. (Borg suggests that some sinners were beyond redemption, chapter 3, n. 16.) In any event, a sinner might be accepted back into society, and might be forgiven, but first the sinner must repent.
What is shocking about what Jesus did is not that he went among sinners, urging them to repent, but that he went among sinners and did not urge them to repent. He accepted them as they were, no matter how unclean, and ate with them and socialized with them. In this respect, Jesus is like the father in the parable of the prodigal son, because when the son returned after living in sin and degradation and shame, the father did not consciously "forgive" the son and then welcome him home. Rather, the father immediately welcomed his son home and showed his son his love by celebrating his return, without even ever asking if the son was sorry or regretted what he had done.
And therein lies the shock (and problem) of Christianity. We are not asked to love sinners in order that they might repent and be saved. We are simply asked to love sinners. Period. And continue to love them as they sin.
Which is very shocking. (And very difficult.)