Yes I do not believe a person can go in and out of salvation but I do believe like the prodigal son they can turn away from God to serve self and come back by the Holy Spirit convicting them if their rebellion . Do you believe that ? I think some people in certain circle call that being a carnal Christian.
I'm glad you asked that, and I believe many Christians misinterpret the parable, and twist it to try to argue "free will". That is a mistake.
Most people take the a parable on its own, out of context. In reality, Jesus told these as a collection of THREE parables:
1) The lost sheep; (Luke 15:4-7);
2) The lost coin; (Luke 15:8-10);
3) The lost son; (Luke 15:11-32);
In all three parables, the owner ALREADY owned the possessions. The Shepherd wasn't looking for "new" sheep, he already had the allotted sheep in His fold, and was simply rounding up those given to him by his master. In the parable fo the lost coin, the woman wasn't simply looking for ANY coins she might find (as some look in couches, or in the coin return of pay phones). She had her OWN coins, and she lost one, and she wanted it back, and she wasn't going to stop looking until she found the coin that was rightly hers.
These teach the doctrine of "perseverance/preservation of the saints".
The third parable isn't going to teach something which is the opposite of what the other two teach. The important thing is that the two sons were ALREADY the sons of their father. The prodigal son was a son before he left, he continued to be a son, and he was received as a son when he returned. The Father wasn't going to receive "anyone", he only received those who were TRULY sons (this touches on the doctrine of election).
What the third parable expands on, and I think many people miss, is that the "meat" of the parable comes from the older son's reaction in vv. 25-32. This isn't explicit, but I think it's justified that the older son represents the Jews, even the Pharisees, and the prodigal son represents Gentile believers. Just as the Pharisees were offended by Jesus accepting Gentile believers, when the Jews were the "faithful" ones, living according to the law their whole lives, and being resentful of those who (to them) lived in "lawlessness", and only came to faith later, the older son was jealous of the younger son because the father chose to give a feast, and cook the fatted calf. The older son felt himself more faithful, but didn't get such a feast. So in a sense, this part of the parable seems to be a rebuke of the Jews who were reluctant to accept Gentile believers, or felt "superior" to them.
I believe John MacArthur's book, "A Tale of Two Sons" goes into more detail regarding this.