This section is again off, what I consider to be, the main topic. To be fair, though, Keller is addressing reasons given against Christianity, and not all of them have to do with the existence of god.
In this section he addresses the complaint that Christianity is too restrictive or selective a community. That they are closed and judgmental. Keller admits that some forms of Christian exclusivity are bigoted and unjust, but says that is not the fault of Christianity as a whole, and that similar fringe behaviors can be found in many larger communities. He also argues that any community must set some limits on membership, or the meaning of the community is lost.
On this we agree. It is clearly necessary to the meaningfulness of a group to have some restrictions on membership. And groups have the right to form, in my opinion anyway, around whatever set of beliefs they want to. I think there are enough important distinctions and points of contention in the population, though, that dividing people up over belief in various mythical minutia is counter productive.
Keller, and others I imagine, would argue that Christianity is a community of people with shared moral beliefs in helping the poor, and loving thy enemy, etc. etc. but in practice, I think, this is not the case. There are many other organizations that are better organized around moral codes, various activist and charity organizations for example, than Christianity is. By this I mean the diversity of opinion of various moral issues within the Christian community will be greater than within those secular organizations with clear moral positions. The practical divisors between Christians and non-Christians are belief, or professed belief, in various stories about Jesus and Mary, which seem a silly thing to form such an fervent community around. This, I think, is the more clearly stated objection to Christianity Keller is hearing, and he hasn’t addressed it, at least not so far.
Still, this is not an argument against the existence of Christianity. I think groups should be able to exist around whatever beliefs they want. It is just an argument against membership in that group.