Note: We cannot speculate on what happened prior of the thief’s life. All we have is the testimony of Scripture to go by here.
First the thief on the cross was saved under the old testament law of Moses. He is not how we are saved today under, the new covenant of Jesus' gospel.
Therefore the thief is no example or proof of salvation by the gospel of Jesus Christ. His will and testament was not yet in effect because He was still alive when He saved the thief. Therefore the thief was saved under the old law of Moses, Hevrews 9:16-17.
Speculation would be the position Fath onlyist, take when they say as fact, the thief was not baptized.
No one can give any evidence that the thief was not obedient to God, before Jesus forgave him, and saved him. Christians sin and forfeit their salvation. To be saved a Christian must repent of sins, 1John 1:9. The thief could have been saved already by John's baptism, Mark 1:4. Then the thief only needed Jesus to forgive him of his sins, which Jesus did.
It is wrong to say, that the thief had no obedience to God prior to his sin of stealing that ultimately led to his crucifixion.
We simply do not know the life of the thief. It is possible he was not baptized prior to being crucified.
Jesus saved him because he(thief) had Godly sorrow for his sins. He believed in Jesus.
To claim Jesus saved him based on faith only is speculation.
It is possible he was baptized,
Mark 1:4-5.
The thief was not saved without obedience.
The thief could have been an obedient jew. We do not know. Very little information is given about the thief on the cross.
What we do know is he had godly sorrow for his sins and he believed in Jesus.
Godly sorrow leads to a change of mind that results in a change of lifestyle,
2Corinthians 7:10,
-For godly sorrow produces repentance leading to salvation.
The thief had to have a sorrow for stealing. If he had no godly sorrow for his sins.
Jesus would not have forgiven his sins and saved him.
The thief was not saved by faith only. He had a repentant heart.
Besides faith itself is a work, John 6:28-29.
Therefore there is no such thing as salvation completely absent from works.
What we need to be focusing on is how we are saved after Christ's death on the cross under His new covenant.
Now all today must be saved by the gospel of Jesus Christ.
That gospel was not commanded when the thief died. It came later.
All examples of Jesus directly saving folks in Matthew, Mark, Luke and John ARE BEFORE HIS GOSPEL WAS IN FORCE.
None of these folks are examples of how we are saved today, read Hebrews 9:16-17.
Acts chapter 2 is when Jesus gospel came into effect. We must be saved the way folks were saved in the book of Acts. That is salvation by the gospel of Jesus Christ.