The one thief on the cross repented after believing in Jesus. This thief on the cross didn't repent right away because he was not yet a believer.
Vs38, "And Peter said to them, "REPENT" and let each one of you be baptized in the name of Jesus Christ; for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit." What's the very first word Peter said to them? "REPENT." that is, to change their minds and fundamental attitudes by accepting the change required. This would produce a renewing of their minds as well as a change in attitude toward sin and self.
The repentant ones could show that change of mind and heart by being baptized in the name of Jesus. Peter is "NOT" teaching that water baptism saves you. The thief was not water baptized but he did repent and that's why Jesus said, "you shall be with Me in Paradise this day." Btw, water baptism does not save you as some churches teach.
IN GOD THE SON,
james