I just finished chapter 4 last night, which wrestles with the challenge of the church perpetrating so much injustice in the world.  Keller approaches the question in a couple different ways:

1. He addresses the common argument that Christian nations have been responsible for war, genocide, slavery, destruction of culture, and a host of other evils.  Keller appeals to the universality of these injustices throughout secular and religious governments alike (using the examples of communist states, imperialist Japan, and few others), and ties this universality to the human propensity for evaluating some belief to supremacy, whether God or an ideal.
2. Secondly, he takes the opportunity to speak about the comparative morality of many within the church, and their deficiency relative to secular counterparts.  His argument leans essentially on one important point of doctrine: common grace.  The idea that all good things flow from God, including those which lead to social and moral stability, is the explanation for how a completely secular individual could appear so much “better” than a Christian.  Coupled with the inherent attractiveness of the gospel of grace to those who are broken, and the long road of sanctification, it is easy to see how this picture could form
3. Finally, Keller deals with the idea of fanaticism by essentially pointing out those who practice the condemning form of faith do not have a full comprehension of the gospel.  He does this biblically through the Sermon on the Mount, pointing out Jesus’ treatment of the “religious”.

The close of the chapter discourses through the idea that the capacity to critique the Christian faith comes mostly from within the faith itself, not outside it.  A purely secular worldview has abject poverty to critique the faith because it is based upon self-driven motivation, which manifests itself in an honor/shame-based culture.  This concept of selfish motivation has no intrinsic motivation to seek justice on behalf of the weak, and it is only because our cultural worldview is deeply built on a historical Christian base that we have any value for the poor and oppressed.

This chapter was an excellent, yet short dialectic that answers many common objections to rational assent to Christian faith.  Keller has a way both intellectually and pastorally explaining his ideas from a position of great humility, and I have appreciated both the soundness and tenor of his ideas.  This is indeed a great book thus far!