There are two very distinct images of the need for conversion in this Gospel passage.
The restlessness, the hunger of the younger son, the rule-breaker. His readiness to sacrifice everything for the satisfaction of those hungers. But human hungers cannot be satisfied with things and with sensations, no matter how exotic and expensive they may be. Sometimes, when that realisation happens, as it inevitably does, the rule-breaker has sense enough to swallow his pride, to turn around and come back, as did the son in this story.
And the elder son, the rule-keeper, was the one who did what he was expected to do, dependably, responsibly. Yet it was his attitude which the father reprimands.
Certainly, the father was not reprimanding him because of his righteousness or his responsibility. The eldest son’s failure was that, while he followed his father’s example on the surface, he certainly did not follow that example in his heart.
In the return of the prodigal, the elder son was called upon not only to do his duty and follow the rules but to go far beyond that, far beyond his own sense of justice, and to be an extravagantly loving and forgiving brother to one who had mocked his rules, broken them all, and, it’s easy to imagine, deeply offended him in the process. He was unable to meet that test, and in that, really, revealed the anger and resentment that had probably marked his righteousness all along.
The elder son was unable to accept the fact that the father’s attitude toward both of them was exactly the same. The father wanted the prodigal son to give up his self-destructive foolishness and come home, and he was overjoyed when that happened. But just as strongly, he wanted the elder son to give up his own brand of self-destructive foolishness, to stop being so angry and resentful, and follow him down the road to welcome the prodigal home.
That is the image of the Father that Christ presents to us in this parable. It is an image that can be a little bit unsettling. It is an image of God to whom punishment, vengeance, recrimination are empty words, a God who asks of us that we simply accept the totally unconditional, loving forgiveness he offers us, and who asks that we offer the same to one another.
It is really just that simple. If we do not learn to forgive one another as lovingly as the Father forgives us, we will probably stay outside. True enough, for us, as for the elder son, it can be a difficult thing to do, that kind of total, loving reconciliation without reservation, without judgement.
But difficult or not, the only other choice is to miss the celebration. And that would be a personal tragedy.