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

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 the compassion of Jesus for those who were crucifying Him [v. 34]) changed his heart and he expressed his faith in Jesus by asking to be a part of His kingdom.