To Luke we owe the story of the criminal who was crucified with Jesus and who asked Jesus to remember him when he entered his (Jesus') kingdom. The import of this story is found not only its moment in the Passion Narrative, but in its joining many other stories of the same nature that very much attracts Luke and fits into his entire story, Gospel and Acts. That is, the call on Jesus for salvation is a theme Luke has insisted on from the very beginning of his Gospel and notes as the essential element of salvation throughout Acts. In preparation for this the angel sang to the shepherds that in Bethlehem there has appeared a savior; indeed, Zachary, just prior to the shepherds' story, sang of Jesus as a 'horn' or power of salvation, one who would defent us from our enemies so that we might live the perfect life of union with God. The calls upon Jesus to save are many in his public life. While he says that he has come to announce the kingdom of God and to call people to repentance so as to enter that kingdom, he often responds to any call upon him for help, for a saving that makes a person better, physically, morally, psychologically. Saving is Jesus' very important characteristic, as far as Luke is concerned. One of the reasons for this emphasis on the person of Jesus as Savior is that, throughout Acts people repeat what had been done so often in the Gospel: "Call on the name of the Lord, Jesus to be saved".
Since Luke writes not just about what happened to Jesus, but about what a disciple can take away as profit from his story, the few lines about the 'Good Thief' contribute to the revelation that, while Jesus will provide what a king provides and while Jesus acts as a divine person, he is, Luke is so proud to say, one who looks to save me from all from which I cannot save myself. The focus, then, is at the moment on the person of Jesus, what he will do, if only I will ask. And I will ask, because I know he is, thanks to his many saving deeds, a savior - indeed, my savior, my only savior.
The good criminal did not think himself worthy of Jesus' attention, and yet he lacked no confidence, because his hope was not on himself, but upon the person he knew to be innocent. Though he did not profess that Jesus was divine and Messiah, he did have a belief that Jesus was a savior. And fortunately he found the right person to ask for salvation.
John Kilgallen, SJ