Keller winds up the Chapter with an interesting observation. Namely, it was the self-centered and licentious sinners who were attracted to Christ. It was the moralistic pharisees who were angered and offended by Him.
"Jesus's teaching consistently attracted the irreligious while offending the Bible-believing, religious people of His day. However, in the main, our churches today do not have this effect. The kind of outsiders Jesus attracted are not attracted to contemporary churches, even our most avant-garde ones. We tend to draw conservative, buttoned-down, moralistic people. The licentious and liberated or the broken and marginal avoid church. That can only mean one thing. If the preaching of our ministers and the practice of our parishioners do not have the same effect on people that Jesus had, then we must not be declaring the same message that Jesus did. If our churches aren't appealing to younger brothers, they must be more full of elder brothers than we'd like to think."
I would not presume to fuss with Keller about this last statement - he is a towering intellect. (Here he defends orthodox Christianity on the campus at Stanford) However, I am not as certain as he about the "mathematical certainty" he ascribes to his conclusion. In other words, does the fact that certain kinds of people are attracted to our churches "only mean one thing?"
On the other hand, I can't really poke a hole in his conclusion and it's worth serious discussion.