I've thought this for sometime, initially because I recall someone preaching in Poland on the older son back in summer 1998, but also from some time studying Luke's gospel a couple of years ago, but Tim Keller has refreshed my thinking about it with some extra clarity.
In Luke 15 Jesus tells three parables. The first two and a half are basically the same thing repeated. Something is lost, and then found and there is great celebration. The odd thing is the chapter should end after that, but it doesn't. It doesn't because the third parable is about a man who has two sons, and there is a whole big bit about the other son. We meet the bad kid who repents of blowing his inheritance and gets salvation at his Father's feast, and we meet the good kid who has never done anything wrong, he however refuses to join the party. Tim Keller helpfully exposes the idolatry of the good kid. He's respectable. He's acceptable. He's sound. He's reliable. And yet he's lost.
What's striking is who all these parables are told to (v1). The sinners are of course there, always following Jesus around. But so are (v2) the pharisees, complaining about the way Jesus welcomes sinners (again). The parables are often applied to the benefit of the sinners - and of course there is good news for them, however lost they are they can come into the feast, heaven will rejoice over their repentance. The shocker is the application to the pharisees who could have it all but miss out on the gospel by their grumbling and good life. There are 'three ways to live', the gospel way, the bad way, the good way (which is really only two, but the 2nd & 3rd need some separate attention).
It's not the first example of this in Luke's gospel, in chapter 10 Jesus hits the same sort of note. He warns his disciples about joy in their evangelistic endeavours (which were pretty impressive) telling them that their hearts should instead rejoice in their salvation (names in heaven, Jesus revealed to them), and then we're shown two good people - the lawyer (who is exposed by the parable of the good samaritan) and Martha who are missing out blinded by the idol of their own goodness and busyness, compared with the third Mary who has got the one thing she needs as she sits and listens to the teaching of Jesus, the soul-satisfying word of God.
Keller also makes an interesting observation about the Sermon on the Mount (I think he credits it to Dick Lucas) which I need to think further about. He observes that the Sermon on the Mount ends with 'two ways to live' - two roads, two houses, two trees. What's the path people are warned off in the SotM? Jesus is one of the ways, what's the other? Not primarily 'bad life' more, warning against pharasaic religiosity, man-impressing piety - all the kind of things that 'good people' love to do. The kind of people who might just cry out 'Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?'. As Jesus addresses his first disciples they have a choice between two ways of living - either they can go with Jesus, or they can go with the sand that is being pious. Life in Jesus leads to life, bears fruit and survives the storm. (Matthew 7) which is a better way than do-gooding.
In February I'm travelling to Plymouth to teach from one of my favourite books, Galatians. I've drawn 4v8-5v1 in their series. That's interesting because the second half is the bit where Paul uses allegory!! Whatever the methodology his point seems to be to show that slavery has already been shown to be futureless, and so they should live a free life in the gospel. The first part of the passage focusses more on the present joylessness of being a slave when they could have the joy of Christ being formed in them by the Spirit. Before hitting that note Paul plays the first part of a melody that might sound dischordant to some, but is really sensational. He accuses the Galatians of going back to their old slavery (4v8).
The thing is, before being Christians they would have been slaves to pagan idols, now as Christians they're enslaving themselves to aspects of the Jewish law. Jewish lawkeepery looks very different to pagan idolatry. It really does. And yet it's actually the same. One puts a religious idol above God, the other puts the idol of religiosity above God. Both are idolatrous. So it was for Peter when he put the idol of a food law above the gospel-created fellowship he as a Jew could enjoy with a Gentile, the day he jumped out of step with the gospel and thus out of step with the Holy Spirit. So too for the Galatians when they bought into circumcision from the silver tongue of their visiting speakers who no doubt had compelling arguments why it would be worth them adding just a little something to Jesus.
Oh, the deceit of our trust in our goodness! Firstly my own goodness is no different from my badness, it's all idolatrous. Secondly, such idolatry is no match for the gospel of Jesus Christ. Jesus is better than it, and his death is enough to provide all the forgiveness required for religious posturing on my part. In an age where people are into spirituality and everyone assumes that if there was a heaven God would of course let them in, we need to preach the gospel to older brothers, good kids, religious ex-pagans, pious environmentalists and self-deceived UCCF team leaders.