It seems to me that the book talks about our failure as Christians to obey Jesus' command to "judge not, lest you be judged". When faced with someone's revealed falleness our response should be to reflect on our own falleness, hidden or not, and not to consider such a person as any worse than our own pitiable selves.
But for the grace of God, there stand I. Now how can I be a vehicle of God's grace to this person. We are saved in community, through the love and mercy of our fellow christians. If we don't see Christ in our fallen brothers and sisters and meet them in their need, we have the parable of the sheep and the goats to teach us where we will eventually find ourselves standing.
Forgive me, as I am the last person who should preach such things to others. Consider this me thinking out loud about my own failure to refrain from judging others.