I believe that the main point of the prodigal son is the same as the main point in the story of the laborers and the vineyard.
“For the kingdom of heaven is like a landowner who went out early in the morning to hire men to work in his vineyard. He agreed to pay them a denarius for the day and sent them into his vineyard. About the third hour he went out and saw others standing in the marketplace doing nothing. He told them, ‘You also go and work in my vineyard, and I will pay you whatever is right.’ So they went. He went out again about the sixth hour and the ninth hour and did the same thing. About the eleventh hour he went out and found still others standing around. He asked them, ‘Why have you been standing here all day long doing nothing?’ ‘Because no one has hired us,’ they answered. He said to them, ‘You also go and work in my vineyard.’ When evening came, the owner of the vineyard said to his foreman, ‘Call the workers and pay them their wages, beginning with the last ones hired and going on to the first.’
The workers who were hired about the eleventh hour came and each received a denarius. So when those came who were hired first, they expected to receive more. But each one of them also received a denarius. When they received it, they began to grumble against the landowner. ‘These men who were hired last worked only one hour,’ they said, ‘and you have made them equal to us who have borne the burden of the work and the heat of the day.’ But he answered one of them, ‘Friend, I am not being unfair to you. Didn’t you agree to work for a denarius? Take your pay and go. I want to give the man who was hired last the same as I gave you. Don’t I have the right to do what I want with my own money? Or are you envious because I am generous?’ So the last will be first, and the first will be last.” Matthew 20:1-16
If the story about the prodigal son were just about mercy, then it could easily have ended with the father throwing the younger returning son a party, but it didn’t because the story wasn’t over.
In the parable of the prodigal son, Jesus presented the son as a character who was disrespectful to his father by demanding his inheritance right then and living as a wasteful drunk and gigolo. The son brought deep shame on his family by his riotous living. By the law of Moses, this son deserved to be stoned.
“If a man have a stubborn and rebellious son, which will not obey the voice of his father, or the voice of his mother, and that, when they have chastened him, will not hearken unto them: Then shall his father and his mother lay hold on him, and bring him out unto the elders of his city, and unto the gate of his place; And they shall say unto the elders of his city, This our son is stubborn and rebellious, he will not obey our voice; he is a glutton, and a drunkard. And all the men of his city shall stone him with stones, that he die: so shalt thou put evil away from among you; and all Israel shall hear, and fear.” Deuteronomy 21:18-21
So instead of the grand welcome home which we’re all familiar with, what should have happened was the younger son be greeted with rocks being hurled his way.
In the setting that Jesus lived in , honor and shame were of much more consequence. Honor and shame affected more than a single individual; the honor or shame a person received was shared by their family. A family’s status was determined by societal rank. If you were a family with honor, you were much more likely to be trusted in life and day to day business; it was easier for you to find a good husband for your daughters, and so on. A family of shame might be mocked and ridiculed, or shunned. The honor or shame of a person very much affected their family’s fate.
So the audience that heard Jesus’ story would have remembered that this son had lived in a manner that deserved death; they also would have realized the shame and stigma that this son had brought on his family. In light of all of this, the elder son cannot be blamed for his attitude and resentment. The younger son had lived in a most vile manner, had shamed his family, and made himself deserving of death. Give this son a party? No way! Not only that, but the elder son happened upon the party! If anyone was deserving of a party, it was him, the loyal, faithful, hardworking son, not this drunken gigolo! The story was intentionally set up so that after hearing the background of the younger son’s actions, the audience would be crying in support of the elder son’s protest. This is exactly the point to which we were to be led, because the main point of Jesus’ story is that it is the right of the father to show mercy his son if he so chooses in the same way that it is God’s right to show mercy to those whom He chooses.
The common theme is God’s sovereignty. The point is best stated by Paul in Romans 9:14, 15:
“What then shall we say? Is God unjust? Not at all! For he says to Moses, ‘I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.'”
God has the right to administer mercy and compassion as He sees fit.
In the story of the laborers and the vineyard, the landowner chastises the grumbling workers who expected to get better treatment than the latecomers stating that since it’s his money, he has the right to decide to do what he wishes with it. He wished to show a kindness and give a bonus to those who didn’t have the opportunity to work all day and so that’s what he did. The point of the story is that just like people have the right to spend their money how they choose, likewise God has the right to show mercy and kindness to those He chooses.