This chapter, in many ways, is a continuation of chapter 10 on sin. Keller discusses the answer to sin, which is Jesus himself. He compares religion (which he describes as salvation through moral effort) with the gospel (which he describes as salvation through grace). He argues that no other religion besides Christianity claims that its leader is the way of salvation. Instead, other religions point to some type of moral effort as being the way of salvation.
Keller states that religion through moral effort leads to a vain religiosity marked by self-righteousness and rule-keeping. This demonstrates that we cannot save ourselves from our sin, and need to accept the grace offered by Christ. He says the acceptance of grace does two things: it motivates us to obey God out of gratitude and it leads both to deep humility and self acceptance. He illustrates this by examining the character Jean Valjean, in Victor Hugo's Les Miserables. He highlights Valjean's transformation from "self-pity and bitterness" to "graciousness toward others" once he has experienced an act of mercy from a priest.
In my Christian life, I grew up experiencing an interesting combination of "religion" and "grace", though I think moralizing was the better part of it. I now attend a church where the gospel, as Keller describes it, is taught and lived out in many believers there. I do get to witness a group of people living lives out of gratitude and they are a blessing to others. When I do see Christianity lived out in a way that makes the community a better place in which to be, it affirms my belief in the gospel. However, I know many can, and have, pointed to lives based on something other than Christianity that have also been beautiful demonstrations of selfless love to others.