Was the prodigal son saved when he left?
Many today use Luke 15:11-32 incorrectly by claiming that the prodigal son was still saved even though he left the Father and went to another country and joined himself to someone else. They claim that he never lost his salvation in the sense that he was always the son of the Father even though he left. The context clearly indicates that son essentially emancipated himself from the Father by his own choice.
In the parable, we know that the son was with the Father at the onset, in His hand (John 10:27-29), saved if you will. The son then removed himself from his safe position under his Father’s care and willfully went to a far country outside of the father's care where the Father could not provide anything for him. We should know the rest of the story where the son "wasted his substance with riotous living" (Luke 15:13) and joined himself with someone other than his Father (Luke 15:15). This is a critical thing to not miss as God does not share His throne with anyone or anything but in this case the son explicitly severed his relationship with the Father and literally joined himself to someone else, a stranger.
This is a clear picture of a child of God going back into sin, exactly what Ezekiel tells us can happen and we should know that it does happen. When someone chooses to do walk away in sin they will die spiritually. The wages of sin is death (Romans 6:23) and the soul that sins dies (Ezekiel 18:20).
Again, When a righteous man doth turn from his righteousness, and commit iniquity, and I lay a stumblingblock before him, he shall die: because thou hast not given him warning, he shall die in his sin, and his righteousness which he hath done shall not be remembered; but his blood will I require at thine hand. Ezekiel 3:20
But we know that the prodigal son was able to repent and return to his Father (Luke 15:20-32) and Ezekiel explains that when one repents and stops sinning then their soul is saved:
Nevertheless if thou warn the righteous man , that the righteous sin not, and he doth not sin, he shall surely live, because he is warned; also thou hast delivered thy soul. Ezekiel 3:21
OSASers chose to believe the first lie told to mankind, that if you are saved and with the Father that you can never walk out of God's kingdom and His care. They claim that one can never forfeit their salvation and die.
And the serpent said unto the woman, “Ye shall not surely die”...Genesis 3:4
We all should know what happened to the first two perfectly saved people who chose to disobey God. It is amazing how many people today are still accepting the lie from the enemy of God.
If anyone chooses to leave God, they certainly can but God has urged us not to throughout His word.
Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. Hebrews 3:12
Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall: 2 Peter 1:10
Full text of the Parable of the Prodigal Son:
And he said, A certain man had two sons:  And the younger of them said to his father, Father, give me the portion of goods that falleth to me . And he divided unto them his living.  And not many days after the younger son gathered all together, and took his journey into a far country, and there wasted his substance with riotous living.  And when he had spent all, there arose a mighty famine in that land; and he began to be in want.  And he went and joined himself to a citizen of that country; and he sent him into his fields to feed swine.  And he would fain have filled his belly with the husks that the swine did eat: and no man gave unto him.  And when he came to himself, he said, How many hired servants of my father's have bread enough and to spare, and I perish with hunger!  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.  And he arose, and came to his father. But when he was yet a great way off, his father saw him, and had compassion, and ran, and fell on his neck, and kissed him.  And the son said unto him, Father, I have sinned against heaven, and in thy sight, and am no more worthy to be called thy son.  But the father said to his servants, Bring forth the best robe, and put it on him; and put a ring on his hand, and shoes on his feet:  And bring hither the fatted calf, and kill it ; and let us eat, and be merry:  For this my son was dead, and is alive again; he was lost, and is found. And they began to be merry.  Now his elder son was in the field: and as he came and drew nigh to the house, he heard musick and dancing.  And he called one of the servants, and asked what these things meant.  And he said unto him, Thy brother is come; and thy father hath killed the fatted calf, because he hath received him safe and sound.  And he was angry, and would not go in: therefore came his father out, and intreated him.  And he answering said to his father, Lo, these many years do I serve thee, neither transgressed I at any time thy commandment: and yet thou never gavest me a kid, that I might make merry with my friends:  But as soon as this thy son was come, which hath devoured thy living with harlots, thou hast killed for him the fatted calf.  And he said unto him, Son, thou art ever with me, and all that I have is thine.  It was meet that we should make merry, and be glad: for this thy brother was dead, and is alive again; and was lost, and is found. Luke 15:11-32