Being brought up in a Protestant, Evangelical church, I found myself in Sunday school from age six. I remember one of my teachers, Mrs. Valcore, telling me that a parable is an "earthly story with a heavenly meaning." That's a decent definition, to be sure, but there's more to it than that.
Skip forward about 50 years, and in a Sunday school class for adults I learned that the purpose of a parable is to get the listener to lay his life alongside the parable in order for him or her to find its application. In other words,
here is the story
here is my life
here is what the story is saying about my life
In light of what it's saying, how is its message relevant to me, if at all? Is there anything I should now do, or say, or feel differently about. If so, how?
In perhaps Jesus' most powerful parable, the story of the "Prodigal Son," Jesus had two audiences. His first audience comprised people who in searching the story for a parallel to their lives identified with the younger son who demanded his inheritance, left home, and then wasted his money in short order, only to be in such want that he had to return home to ask his father for forgiveness.
The other audience comprised people who probably didn't identify with either son. If they identified with anyone in the story, it was likely the father because he appears to have been cursed with a wayward and ungrateful son.
Toward the father they may have felt compassion.
Jesus' purpose for the latter audience, however, was to get them to identify with the elder son because he kept his nose clean, worked hard, respected his father, and had only contempt for--and possibly some envy of--his younger brother for having his cake and eating it too, as it were.
Quite likely the majority of the latter audience, composed as it was of hyper-vigilant Pharisees and scribes (see Luke 15:1-2), wouldn't identify with the older son in the way Jesus wanted them to identify with him until after they had mulled the story over in their minds for awhile.
Eventually, some of them (a remnant? a minority?) would be convicted of having the same holier-than-thou attitude of the older son and might just repent of their self-righteousness. For some others of them, Jesus' point would have sailed right over their heads, and their identification with the older son would have stopped with the notion that the older son was righteous before God and had no need to repent of anything.
Now perhaps you can understand why Jesus spoke in parables. Upon hearing Jesus' story of the two sons and their father, only those listeners who recognized their need to repent, as did the younger son, would be welcomed back into the bosom of the heavenly Father. Those listeners who failed, as did the older son, to recognize their need to repent would be at odds with the heavenly Father until they confessed their sin of self-righteousness to him.
Jesus knew that God's word as it came from his lips to his listeners ears would not return unto God void or empty (see Isaiah 55:11). For some of Jesus' listeners the return would be immediate in the form of a changed life, though some would fall away for various reasons (see Jesus' parable of the sower in Matthew Chapter 13).
For others of Jesus' listeners, the return might come much later or perhaps not at all. For either category of listeners, however, God's word would accomplish its purpose, whether that purpose was to grant repentance to a sinner or to confirm a sinner in his sin.
Such is the power of God's word, whether it is presented in a discursive manner or whether it is packaged in story form. Jesus was a master teacher in both modes. Moreover, his teaching, when believed, brought life; when not believed, death. And so it is with faithful Christian witness, preaching, and teaching today (see 2 Corinthians 2:14-17).