Was the Prodigal Son a son before he left? Why must this be a parable about salvation and not the repentence of a son? Why do you define doctrine by parable and then make the statements of doctrine fit the parable instead of defining doctrine by statements of doctrine and then reading the parables appropriately?
Matthew 18:12-13 "What do you think? If a man has a hundred sheep, and one of them goes astray, does he not leave the ninety-nine and go to the mountains to seek the one that is straying? And if he should find it, assuredly, I say to you, he rejoices more over that sheep than over the ninety-nine that did not go astray.
Because they are both parts of the whole and we lack the knowledge to give them differing weights according only to human reason.
You cannot know the mind of God therefore you cannot know the methodology that He uses to determine salvation. That's the difference between us. I accept this and you reject it.
He did send His Son and Holy Spirit to lead us to the right path, that much we know.