The thief lived under a different covenant and was forgiven under a covenant that ceased at the death of Jesus. Upon His death the First Covenant ended (the Law of Moses) and the Second Covenant was instituted (Hebrews 9:15-17; 10:9-10). The command to be baptized in water to obtain the forgiveness of sins is found following the death and resurrection of Jesus Christ (Mark 16:16). Since the death and resurrection of Jesus, all who live, are accountable to the instructions found in the New Covenant. This is a universal covenant for all mankind (Mark 16:15; Matthew 28:19). Included in this covenant is the command to be immersed in water for the remission of sins (Acts 2:38; 8:35-38; 10:47-48; 18:8; 22:16; Ephesians 4:5; Colossians 2:12-13; 1 Peter 3:21). In addition, it is purely assumed that he wasn't baptized. He may have been baptized under John's baptism, for he knew about the Kingdom of God (Luke 23:42).