The conundrum of personal identity is one of the perennial issues in philosophy. Even though you change over the course of a lifetime, changing in a multitude of little ways and big ways, are you the same person throughout it all?
It's difficult to give a clearcut answer to this question because it boils down to degrees of continuity, similarity, and dissimilarity. To questions of what is intrinsic or extrinsic to what, if anything, makes you the unique individual that you are.
Let's take some concrete examples. On his deathbed, there's a sense in which Paul was the same man responsible for the death of many Christians. Paul did it in a way that, say, Matthew Henry did not. And, indeed, Paul himself was always haunted by his preconversion misconduct.
On the other hand, there's a sense in which Paul could honestly say, "I'm a changed man. I'm not the same man who did those terrible things 30 years ago."
In a way, that's true. If Paul could go back in time, knowing then what he knew now, he wouldn't do the same things. He couldn't do the same things. His beliefs changed. His attitude changed.
If you put him in a time machine and sent him back, if he met his younger self, and they met face-to-face talk, there's a very real sense in which the man who returned from the future is not the same man as his younger counterpart in the past. It's inconceivable that Paul, as an old man, would do what he did as a young man. As an old man, he was a gracious, saintly, long-suffering Christian. He would never persecute Christians. He would never arrange for their death. There's be no meeting of minds between his younger and older self.
Now let's take a very different example: suppose a Nazi official escapes to South America. He was responsible for the death of thousands of Jews. And not just because he was following orders. He was a Nazi enthusiast. He took pride in his work.
Decades later, Mossad agents discover him in a nursing home. Normally, they'd smuggle him out of the country to face trial in Israel. But he's senile. He no longer remembers his crimes. Is he now the same man who sent innocent Jews to the gas chambers?
It's a quandary. On the one hand it's unjust that he eluded justice all these years. On the other hand, it seems unjust to try him now that his mind is gone. They caught up with him too late. Yet that, in itself, is unfair. He got away with it. At least in this life. Of course, even if he had all his marbles, they can't really punish him adequately for the evil he did.
At the same time, this goes to ambiguities regarding personal identity. From a dualistic standpoint, his mind is still intact, but smothered by a deteriorating brain. Once he dies, once the soul is decoupled from the brain, his mind and memories will resurface.
Take pain. If you're in excruciating pain, you can't think. It blocks reason. It consumes consciousness. But once the pain is relieved, reason returns.
It's like saying I lost my keys. There are roughly two different ways in which I might "lose" them:
i) I can lose my keys even though they are still in my possession. I might absent-mindedly put them in a drawer, then forget where I put them. In a sense I still have them, but I can't access them because I don't remember where I mislaid them. Even if I remembered putting them in a drawer, I don't recall which piece of furniture.
But just as I accidentally misplaced my keys, I might accidentally discover them if I happen to be rifling through that drawer for an unrelated reason.
ii) I can lose my keys if they fall out of my pocket. Or I take them out, put them on the counter while I'm trying to locate something in my pocket, and neglect to put them back.
That might happen in a public place. When I go back to the car, or walk back to the house, and reach for my keys, I suddenly realize they're gone! I run through my mind where I've been. Where might I have left them?
But even if I go back to the right place, they may be gone. Someone took them. They are lot forever. I will never see them again.
Eschatological justice is more like (i) than (ii). Even if, due to senility, the Nazi fugitive isn't the same man at the time of his death, he will be restored–to stand trial before the bar of God.
But for Paul, his sins were forgiven.