How could one of the thieves on the cross be saved without being baptized?
The salvation of one of the thieves on the cross is recorded in Luke chapter 23. First, only one of the thieves asked Jesus for His mercy; the other was unwilling to repent and ask. In Luke 5:24 we are told that Jesus had the power to forgive sins while He was on earth. Equipped with this authority, Jesus forgave a number of people of their sins. After the thief asked Jesus to remember him when Jesus came into His kingdom, Jesus replied, “Today you will be with me in paradise.” It was not until the day of Pentecost that people would be able to obey the command to be immersed in water for the forgiveness of their sins. The story of the thief on the cross was not given to us as an example of how we are saved today, any more than the story of the woman caught in adultery (John 8:1-11) or the paralytic man (Matthew 9:1-8) were. These examples were given to demonstrate the compassion and authority of Jesus, even as He was later on the brink of death Himself.
