When studying the Bible with members of denominational churches and the subject of baptism comes up, they will at some point in the study invariably bring up the thief and the promise that Jesus made to him concerning being in paradise that day.  The story of the thieves is found in all three synoptic gospels. It is found in Matthew 27:38-44, Mark 15:27-28 and Luke 23:32-43.  

It is only the account in Luke that our denominational friends are concerned about.  You will note that one of the thieves condemns the other for joining the people in railing against Jesus.  He then asks Jesus to remember him when He comes in His kingdom.  It is then that Jesus promises him that that they will be together in paradise that very day.  Our friends will point out that the thief wasn't baptized, yet he was saved therefore baptism could not be necessary for salvation.

There are fallacies in their proposition.  First is that Jesus had not yet given the commandment that we be baptized in order to receive salvation.  That didn't happen until after He was resurrected and just before He ascended back into Heaven.  In Matthew 28:18–20 (KJV 1900) we read  ^"And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. ¹⁹ Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: ²⁰ Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen."  And in Mark 16:15–18 (KJV 1900) Mark writes "And he said unto them, Go ye into all the world, and preach the gospel to every creature.  He that believeth and is baptized shall be saved; but he that believeth not shall be damned. ¹⁷ And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues; ¹⁸ They shall take up serpents; and if they drink any deadly thing, it shall not hurt them; they shall lay hands on the sick, and they shall recover."  Because Jesus issued these commands after the thief had died and had gone to paradise with Jesus, he wasn't subject to these commands.  Therefore he was not required to be baptized for his salvation, unlike us.

Second, Jesus had authority while living on Earth to forgive men of their sins.  This is well documented in Matthew 9:2-8, Mark 2:5-12, Luke 520-26, and Luke 7:44-50.  Because of this Jesus had the authority to forgive the thief of his sins, which He did in promising that they would be together that very day in paradise.  Next Jesus didn't tell the thief to repent or confess either, neither of which the denominations deny are required for salvation.  Consider this, it would have been impossible for the thief to have made the confession that we are told we need to make in order to be saved.  In Romans 10:8–10 (KJV 1900) the Apostle Paul writes "But what saith it? The word is nigh thee, even in thy mouth, and in thy heart: that is, the word of faith, which we preach; That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation."  Since the confession that saves us is that we believe that God raised Jesus from the dead, the thief could not make that confession because he died before Jesus was resurrected. 

Because of these points the story of the Thief and Jesus does not teach us that baptism is not necessary for salvation any more than it teaches that we can go to heaven without repentance nor confession.