The thief said to Jesus “remember me when thou comest into thy kingdom.” What did the thief really know about salvation? From the text, we have no reason to assume that he knew much of anything.
The profession of faith made by the thief on the cross is fairly weak. If someone offered this “prayer” at a gospel meeting we would try to clarify the gospel for them to make sure they understood. If the testimony of saying “remember me” was all we had to go on, we would likely doubt the legitimacy of his salvation. If an evangelist asked him the question “how do you know you are saved? there is no way he would get a pass by answering “I asked Jesus to remember me when He comes into His kingdom.” What about sin, sacrifice, atonement, faith, grace, etc? And yet this thief is one of the few people in Scripture that the Bible insists is in “paradise.”
Although the thief seemed to have only a remedial understanding of the gospel, he does have one of the greatest testimonies of repentance ever. He is the only person I know of who may have never sinned after being saved.
Some people question their salvation and will look back to their moment of profession for confirmation. They will review the doctrines just to make sure that they really get it. Maybe instead they should just stop living like a sinner.