In Chapter two of “The Prodigal God”, Keller helps us think through the story of the younger and older lost sons more deeply.
The younger son certainly does make a bold request. And yet the father willingly grants it! What a devastating loss it would have been for the father to give up his property and wealth for his son. It is helpful that Keller points out the greek word for ‘property’. It literally means life! I don’t have any children, but look forward to the day I do. However, I can only imagine the knife that would go through my heart if a son of mine (let’s hope this never happens!) boldly walked up to me and asked for his share of my savings and the house... I wouldn’t know how to react. I know I certainly wouldn’t agree to his request!
At least this younger son realises his mistake, once he’s squandered all the money. He even comes up with a plausible plan. He could work as his father’s servant to pay off his debt. And yet, after all that, does it not make us wonder even more at the depth of the father’s love; that he should freely and willingly take this lost son back. No questions asked, no bargaining together, no conditions for his acceptance back into the family. The father opens his arms to him. And he doesn’t just give him his room back and say “there you go”. He puts his best robe on him, throws him a huge party, and restores him to his previous family standing.
As you ponder this parable for the third, or maybe the three hundred and third time, I hope the depth of God’s love for you and the forgiveness he gives still amaze you. Don’t let that become old news. The parables surrounding this story in Luke chapter 15 (the parable of the lost coin and the parable of the lost sheep) give us an even bigger picture of god’s love and forgiveness. They shout to us of how precious the lost are to God, and how wonderful it is when they are brought back to him. Our great God freely loves, and freely forgives. As Keller says so well, “God’s love and forgiveness can pardon and restore any and every kind of sin or wrongdoing. It doesn’t matter who you are or what you’ve done... There is no evil that the father’s love cannot pardon and cover, there is no sin that is a match for his grace.” p24
The response of the older brother, then, becomes so much more shocking in light of the father’s response to his younger son. And yet in the older son’s attitude and actions, we realise he is just as lost as the first son. The older son is only concerned for his own reward, and for his own justice. The older brother demands what he sees as his rights from the father; benefits and a share in his father’s wealth almost as a payment for his strict obedience. While the father offers the older son love and an invitation to join the party, we a left without a final response from this brother.
I love the fresh look we get at sin in chapter three. 
By understanding these two sons more fully, Keller shows us how to more fully understand sin.
While Keller looks at the example of both sons, I’m just going to think about the example of the older brother, as I think that is slightly less well-chartered territory.
Keller shows that the older brother in this parable is representing the Pharisees and their strict observance of the law. And this parable says something striking. As Keller says, ‘the lover of prostitutes is saved but the man of moral rectitude is lost.’p34.
The elder brother’s sinfulness is in his obedience; because his obedience is moralistic and dutiful. It is obedience without heart. The barrier between the older brother and the father is this son’s pride in his moral record. He expects reward and wants it on his own terms.
As Keller says towards the end of the chapter, ‘Sin is not just breaking the rules, it is putting yourself in the place of God as Saviour, Lord and Judge, just as each son sought to displace the authority of the father in his own life’ p43.
I think this has implications for both Christians and non-Christians.
Maybe this way of thinking is how your non-Christian friend thinks about God. If they’re just good enough, enough of the time, God will let them into his great feast, won’t he? Maybe if they work hard at keeping the laws of the land, respecting people, and praying every now and then to God, he’ll be happy with their ‘righteous’ life and accept them into his kingdom.
Well we need to keep exposing the amazing and shocking gospel of grace to those around us who think this way. Only then will they see there is nothing they can do to be good enough for god and be accepted by him.
I wonder how often we as Christians slip into thinking like this older brother about our ‘righteous’ lives. We forget we’re already saved, but instead think that because we’ve perhaps suffered a little as Christians, or because we’ve remained faithful and served God for many years, we deserve to be accepted by God. We might even slip into thinking we’ll be ok on judgement day because we’re doing so well at following Jesus. Not actually because of what Jesus did! We look down at those ‘sinners’ around us, and congratulate ourselves for being so holy.
In essence, like the Pharisees, those of us thinking this way have totally lost perspective of ourselves, and of God. We have forgotten grace, and are instead trying to work for our own salvation. This is completely the opposite of what God wants! Though we try to make ourselves morally upright, we know that cannot happen. We cannot ever make ourselves morally good enough to be worthy of any reward. Our ‘righteousness’ will just never be ‘right’ enough.
Let us instead cling to God’s grace. Let us, unlike the older brother, remember that we are in need of this grace. Let us not think of what we can do, but instead, of what Christ has done.
The author of one of my favourite hymns says it better than I ever could.
Not the labours of my hands
can fulfill your law's demands;
could my zeal no respite know,
could my tears forever flow,
all for sin could not atone;
you must save, and you alone.
Nothing in my hand I bring,
simply to your cross I cling;
naked, come to you for dress;
helpless, look to you for grace;
stained by sin to you I cry;
wash me, Saviour, or I die.
Augustus M. Toplady 1776
 That might not have come out the way I meant it! I’m not enthusiastic about sin, just glad of a chance to understand sin and our own sinfulness better with Keller’s help.