I disagree with you. Here's why.
The thief WAS NOT saved by the gospel of Jesus Christ. He did not have the faith required by the gospel, like we do, which is to believe that Jesus died, was buried and raised from the dead..
Before He died on the cross, Jesus forgave people directly as He chose. Since His death, we must meet the terms of His will to be forgiven.
I am not saying that the thief didn't have faith. I'm saying he didn't have the faith which is required of us in the gospel of Jesus Christ. The thief could not believe in the resurrection of Jesus because it hadn't happened yet.
The truth is that the thief was not saved under the same law and dispensation that we are under. He was still under the Old Testament, not the gospel. That is why the thief was not required to believe what we must believe nor to receive the same baptism that we must receive. The terms of his forgiveness teach us nothing about what we must do to be saved.
Jesus' death, burial, and resurrection are the fundamental facts of the gospel that we must receive, believe, and hold fast in order to be saved. The thief on the cross could not possibly believe Jesus had been raised from the dead, because He had not yet died, let alone been buried and raised.
Romans 10:9 - If you confess with your mouth the Lord Jesus and believe in your heart that God has raised him from the dead, you will be saved.
So, to have the faith required by the gospel, we must believe that God "has raised" Jesus from the dead.
The point is that the thief was saved before the terms of the gospel came into effect. So we cannot use his case to prove one way or the other what people must do today to be saved.
We have two excellent examples of "WHAT WE MUST DO TO BE SAVED" in Acts 2:36-41 and Acts 16:25-34.
A reasonable point.
My analogy, like any analogy, breaks if pushed too hard.
I see the irrefutable core of the Gospel and salvation to be a trust in Jesus.
Anything that removes Jesus from 'Front and Center' creates dangers.
Where I have struggled to wrap my brain around 'TRUTH' is how much is enough.
To give a slightly silly, but useful example ...
There are those (few in number) who place so much emphasis on Baptism that it becomes a Prerequisite for salvation.
And not merely any baptism will do.
You must be baptized by the right people (those with the authority, given by someone who received the authority, from someone who in turn had the authority, to empower the baptizer).
You must be baptized in the correct way ... three times (once in the name of the Father, once in the name of the Son and once in the name of the Spirit).
I forget whether it was three times forward or three times backwards ... but I remember that they claimed it mattered.
Anyone not properly baptized was, in their opinion, not saved.
... this uses hyperbole to point out a far more legitimate concern for me.
Now much is enough?
Can a death-bed confession of someone dying of C.O.P.D. ... who cannot walk and can barely whisper ... lead to salvation?
Can the Gospel Truth be presented, understood and believed sufficiently to obtain the Grace that Jesus secured (as you said, through his life, death, and resurrection)?
I have trouble imagining someone standing before the Judgement Seat and being disqualified from eternal life for a point of technical ignorance ("sorry, you were sprinkled and salvation requires complete immersion").
My mentor taught me that Living the Christian faith means surrendering everything that you know about you TODAY, to everything that you know about God TODAY.
Tomorrow, you will know a little more about you and a little more about God.
IMHO, the Thief met that prerequisite.
IMHO, damnation (expulsion from the presence of God) comes when you know that God requires something and that you should do it (or shouldn't do it) ... but choose to reject God and do things your way.
Those who want no part of God ... are granted their wish.
Those who want a God other than the one who has revealed himself to them ... are granted their wish.
I could be wrong (it would not be the first time), but that is my current working hypothesis for how 'salvation' works.