The Thief on the Cross incident in the New Testament (Luke 23:39-43) is often cited as a proof text of how one is appropriated salvation today. That all one has to do is believe in Jesus with faith to be saved. In fact this is not the case as stated in Hebrews chapter 9 in reference to Jesus will being in force upon His death on the cross, ushering in the New Covenant and teachings of His Apostles beginning in Acts chapters 1 and 2.
The serious Bible student will find that this thief and Jesus while still alive were under the Law of Moses, the Old Covenant, which passed away by being nailed to the cross forever (Colossians 2:14-15). That the thief and his contemporaries were in the middle of a transition point of history during the unfolding of Gods plan for the future of His church. It is now the full Gospel as given by Jesus to the Apostles (Acts 1:2) beginning with Apostle Peters first sermon in Jerusalem that is in force and must be obeyed in order to have Salvation. This short introductory video lesson will help in your studies.