Everyone knows the story of "The Prodigal Son" as it is so commonly coined in most every translation of the bible you will find. It very well may be the best known story in the world outside of the Good Samaritan. Everyone sees the metaphors of sin, redemption, grace, and the love of the father and,understandably, feel all warm and tingly inside when recounting God's love for us in receiving us as sons in our sinful state. It's a beautiful picture and a great lesson to be learned.
But that's only half of the story. Tim Keller reveals the second half of Jesus' story he prefers to call "The Parable of the Lost Sons". While he does bring up the facts and application of the traditional understanding, most of Keller's focus is on "Act 2", the sin and isolation of older brother. Keller states that we all have dispositions and temperaments that predispose us to a life of moral conformity or self-discovery. Sadly, I had to read this book knowing that my propensity is toward morality, judgement, and disdain, characteristics that aim me toward the elder brother -- the one that didn't enter the feast (a metaphor for the Heavenly celebration throughout the Scriptures that Keller also discloses).
The big idea as I gathered it, without giving away the entire book, is that while both elder brothers and younger brothers are in sin and disconnected from the father, the plight of the elder brother is perhaps more dangerous than the separation and rebellion of the younger brother. Younger brothers know that they are disconnected from their father and willfully disobey. Elder brothers assume their relationship and inheritance, blind to their own iniquity; this is a very dangerous place to be. As Keller says, "If you know you are sick you may go to a doctor; if you don't know you're sick you won't--you'll just die." If that's not sobering, I don't know what is.
By putting a flawed elder brother in the story, Jesus is making us yearn for a true one. Keller tells us that this comes in Christ who does not stay and take care of his inheritance but departs the father, searches and finds his younger brother, and then returns him to the family at his own expense.
I will stop there and leave the rest of the book for you're reading. It's a short and easy read and one that I would highly recommend. Keller's writing style is quite enjoyable. He speaks on a user-friendly level and uses a lot of great references to other short stories and movies to drive his points home well. If you're curious about the title, you should look up the definition of prodigal. Keller doesn't walk you through it, but it's quite profound, really.
In closing, Keller challenges us all by saying, "If our churches aren't appealing to younger brothers, they must be more full of elder brothers than we'd like to think." It's something we should all consider as we fight the default mode of the human heart, religion.