Note 3 at Lu 23:40: Only Luke gave us the account of one of the thieves defending Jesus and receiving salvation. Even in the midst of terrible agony, Jesus was reconciling unto God those who would receive it.

Because Matthew said "the thieves" (plural) were mocking Jesus, it must be that this malefactor, who asked Jesus to remember him when He came into His kingdom, mocked Jesus at first. However, something (possibly Jesus' compassion for those who were crucifying Him [Lu 23:34]) changed his heart, and he expressed his faith in Jesus by asking to be a part of His kingdom.