Keller writes about the Parable of the Prodigal Son (as we've generally come to know it), "Jesus' purpose is not to warm our hearts but to shatter our categories. Through this parable Jesus challenges what nearly everyone has ever thought about God, sin, salvation." (P. 10).
He calls this a drama in two acts: 1. The lost younger brother; 2. The lost older brother.
1. The original listeners to Jesus' parable would have been appalled by the request of the younger brother. This would have been a request of the highest disrespect to the father. The father is treated as merely a means to the end of enjoying his wealth. That's all he means to the youngest brother - until he means more. It's surprising that the father doesn't merely discipline the son. To fulfill his request, he would need to sell off a portion of his property to turn it into cash. The father endures this all - the loss of honor and the rejection of his love. The father maintains his affection for his son through it all. When the time comes, the younger brother does not need to earn his way back into the affection of the father. He never lost his father's affection. The father's love overflows through the fattened calf for the son to a meal, a celebration, that the whole village will enjoy.
2. The older brother is furious. In his anger, he, too, disrespects his father in the way that he rejects the invitation to the party and the way he addresses him. In the same way that the younger brother saw the father as merely the moneybags, the older brother does too. And when the inheritance that is now solely his is gleaned from to celebrate the younger son, he is angry - angry at the cost to his inheritance. The two are alike. Neither simply loves the father for the love the father has for them.
The younger brother gets it. The question is left whether the older brother does, but it doesn't look good for him. Keller finishes the section on the two brothers saying, "Jesus is redefining everything we thought we knoew about connecting with God. He is redefining sin, what it means to be lost, and what it meas to the saved" (p. 28). Neither of these paths will lead to salvation. Both are dead ends. There is another, better, way.