We actually know little about the salvation of the thief. Whether he knew anything of salvation from sin through Christ would be difficult to prove. In fact, whether or not he had earlier been baptized by John the Baptist and since that time fallen into sin would be of question (Matthew 3:1-6). Without quibbling over these incidentals, the salvation of the thief was miraculous and none can be saved that way now. The New Testament teaches that while Christ was on earth He had the power to forgive sins, as He chose. To the man sick of palsy Jesus said, Thy sins be forgiven thee. Certain of the scribes questioned His power to do this, and Jesus replied, But that ye may know that the Son of man hath power on earth to forgive sins, (then saith he to the sick of palsy), Arise, take up thy bed, and go unto thine house (Matthew 9:1-6). Christ had the power to forgive sins on whatever conditions He chose while on earth, but He had laid down conditions of salvation to take effect after His death which will last throughout all generations. The Hebrew writer gives an illustration which explains, For where a testament is, there must also of necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testaor liveth (Hebrews 9:16-17).A man’s last will and testament is not fixed until after his death. As long as he lives he sustains the right to change the gifts or even the beneficiaries. But after his death, the will is sealed and cannot be changed even by the courts of the land. So is the will of Christ; it became of force after His death.
To illustrate, consider a man who has two sons, and he stipulates in his will if each son is twenty-one years of age and married he shall receive his portion of the inheritance. But if shortly before his death the father finds some reason to favor one of his sons, and he gives the inheritance to him before he is married, does not the father have the power (right) to do this? We can also understand that the other son will not receive of the father until he is both twenty-one and married even though his brother was granted such without marriage. After the death of the father the will becomes effective and cannot be altered.