This nameless criminal had joined his fellow in mocking the suffering Saviour (Matt. 27:44). A short time later, he changed his outlook and instead rebuked his former partner in crime. What brought about the change cannot be fully known. Yet we can observe some details that Luke mentions.
First, the condemned men knew in a vague way initially what it meant for Jesus to be the Christ. We can see that was the case from the way the other criminal spoke to Jesus (Luke 23:39). He joined together the word 'Christ' and the concept of salvation when he asked that Jesus would save them from the cross. Of course, he may have been mocking, and he only wanted to be rescued from crucifixion in any case. Still they linked together the Messiah and salvation, which is a pointer towards them having some grasp of the message of the Old Testament. And the penitent criminal became aware that Jesus was the Messiah with a very different salvation on offer.
Second, the criminal who repented had been thinking about God, and thinking about him as the judge. We can see that was the case from the way that he addressed his fellow-criminal. He appreciated that the proper response towards God was one of reverential fear. In addition, he realised that in a short time he would stand before God as Judge and give account for his actions. And he knew that condemnation for his actions would be completely just as far as God was concerned.
Third, the penitent criminal recognised the uniqueness of Jesus when he stated that he had done nothing wrong. He came to the same conclusion as did the centurion later because they both stated that Jesus was innocent. One assumes it was connected to the demeanour of Jesus on the cross. People behave usually according to their circumstances. If an individual finds himself in a place of cruelty and pain from which there is no escape he will show his response by his angry and desperate words. Yet the criminal had observed that Jesus behaved totally differently. Instead of hatred, there was love revealed when he prayed for the soldiers. The criminal heard words that would have indicated to him that the One suffering beside him knew how great sinners could be forgiven.
Repentance occurs when we grasp that Jesus is the Saviour of sinners. It follows the realisation that we will give an account of our lives to the God we have offended. And a penitent person recognises the great difference between his imperfection and the perfection of Jesus.
Did the dying criminal put those three things together – Jesus being the Christ, his fear of God the Judge, and the prayer of Jesus for the soldiers – under the guidance of the Holy Spirit? There could have been many other aspects to his case, but they all led him to make an incredible request.