In the Bible, in John 21:15-17, when the risen Jesus meets with his disciples, he repeatedly tells them to feed and tend his sheep and lambs. This is consistent with many other New Testament verses that suggest that it is the duty of the Christian to care for the weakest and most vulnerable in our society. Not to do so, and to ignore the needs of the sick and hungry and poor is described in Matthew 25:40 as a sin against God ("Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me.").
Thus, when the wealthy and middle classes regard the ghetto as someone else's problem, or not their fault, they are sinning, in the sense of not fulfilling their moral responsibilities. The existence of places in which drugs, prostitution, abject poverty, untreated illness, and other such ills exist is not just an abstract social problem from Kozol's point of view, but a sin of omission, in which people are just as guilty through neglect or complicity as they would be if they were abusing children or selling drugs themselves.