I will arise and go to my father, and will say unto him, Father, I have sinned against heaven, and before thee, and am no more worthy to be called thy son: make me as one of thy hired servants. Luke 15:18, 19.
At least part of the lost son's theology is correct. Truly unworthy to be called a son, he had lived in a state of rebellion, beginning with his command to his father to give him his share of the inheritance. "I want mine now, old man. I can't wait forever for you to die. I've got a life to live. And I want to do it while I'm young. So cough it up because you can't take it with you."
Nice kid! Just the kind most of us never want. Disrespectful and selfish.
And then he got his wish. And with the wish came dissipation in the form of substance abuse, sex on the go, and all the other pleasures of the world. And he really had no desire for the father as long as he had money to support his habit. He turns to the father only when he is desperate. Not much love there, only driving desperation. Yes, he is truly unworthy of being a son. But at last he is willing to admit it.
And he's right on another point: "I have sinned against heaven, and before thee." Sin is not merely against other people. It is primarily against God, the Father of us all. David expressed that truth after his own "far country" experience of adultery with Bathsheba and the murder of Uriah to cover his tracks. In the end, in repentance he cried out to God, "Against thee, thee only have I sinned" (Ps. 51:4, RSV).
Good theology so far. But then the younger son goes off the track. "Treat me as one of your hired servants" is his plea.
To understand the implications of that request, we need to remember that there were three levels of young men in a prosperous household. At the top of the social heap were the sons. They had rights and privileges that no one else had. Beyond that, they were heirs. But the returning boy knew that he had forfeited that position.
Then there were the slaves. They had some security. After all, the family owned the slaves and thus they belonged to the household. At the bottom of the pile were the hired servants. Here today, gone tomorrow, depending on the need for workers. The most insecure position.
In effect, the son is going to request that the father give him exactly what he deserves. Apparently he desired to work his way back into good favor. By taking the absolutely lowest spot, perhaps through years of hard work he could make himself "worthy" of sonship again.
But in that "salvation by works" approach, he totally misunderstood the Father.