I like the example of the prodigal son because the story's main concept concerns the fact that he is his father's son, evinced by his overwhelming desire (Holy Spirit's work--Gal 5:17) to return to fellowship with his father. It does not question the concept of him being the father's son, which I believe portrays that once union is established it is inseparable, which is not the same for fellowship.
Just as one born will always be a son to his father, even though there is no fellowship (close union) between them, they are still in a father-son union genealogically. One can be in union with God but not in fellowship, which fellowship is always eventually restored. There are examples of an appearance of a union, which union is eventually evinced to never have existed by the fact of the desire of a permanent absence of fellowship.
Unlike the natural union of a family where, though they are genealogically united, they can remain out of fellowship, the union with God will ultimately be evinced by the restored fellowship, which without would leave only one alternative--there was no union to begin with!
Either one is a Christian, which means being born again, or they have never been a Christian. All newborn Christians have much to learn concerning evidences of regeneration and applied Bible doctrine, whose lifestyle will progressively evince that God is working in them. Does not God work in every believer the unfailing desire for His will (Phil 2:13) and maintain it by His Spirit (Gal 5:17). If their lifestyle eventually evinces that God is not working His will in them, it confirms that regeneration has yet to occur.
It's not sensible to conceive that God would give salvation to anyone, knowing it would be temporarily. If He knew there was a chance of it being temporal for any reason, why would He give it; and more so, why is it called "eternal salvation" (Heb 5:9) if it is not permanent for the individual, for if it's not permanent in one's life it was something other than salvation, because there is only one type of salvation--eternal salvation. If one could get out of eternal salvation then it wasn’t eternal, which is conflicting. It's the "eternal" aspect of salvation that gives its greatest value and definition, without which would be worthless and of no use.
The primary block in understanding that "the gift of God is eternal life" (Rom 6:23) isn't because one is not a believer but is due to being brought up in the Christian life under the error of thinking that the recipient of salvation owes God for it, which concept understandably results in the desire to somehow pay for it. But this leaves the believer in heresy, unaware of the meaning of "gift" (free). God’s grace endowed does not incur debt to Him, but to "one another" (Rom 13:8), which left unpaid is "as one that beateth the air."
Such is the example of the story of a man who gave someone a luxurious display cabinet as a gift and while he was leaving he happened to notice the person was about to apply sandpaper to it (because he felt obligated to do something for the gift), upon which he told the man, "There's nothing that needs to be done to the cabinet, because it is finished." (John 19:30).
Granted, Scripture contains words and phrases that reveal temporal situations which appear to be related to the possession of salvation but are rather those which are related to a false pursuit, but not the possession of salvation.
Every aspect of salvation can be known and used in the service of God, but remove its eternal element, which is its pinnacle aspect, then suddenly you have a fallen disposable structure of “wood, hay, and stubble” (heretical doctrine) instead of “gold, silver, and precious stones” (the Gospel of Christ).