One common argument against the need for water baptism is the story of "The Thief on the Cross." Here is the scripture:
LUKE 23:39 ¶ And one of the malefactors which were hanged railed on him, saying, If thou be Christ, save thyself and us.
LUKE 23:40 But the other answering rebuked him, saying, Dost not thou fear God, seeing thou art in the same condemnation?
LUKE 23:41 And we indeed justly; for we receive the due reward of our deeds: but this man hath done nothing amiss.
LUKE 23:42 And he said unto Jesus, Lord, remember me when thou comest into thy kingdom.
LUKE 23:43 And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.
So upon this basis, when told of water baptism, they answer: "What about the thief on the cross? He wasn't baptized, and yet Jesus said that he'd be with Him in paradise." From this, they conclude that baptism is not required for salvation, only belief is required.
The thief on the cross has nothing whatsoever to do with water baptism, either pro or con. When Jesus made that statement to the thief on the cross at Luke 23:43, Jesus had not yet died for anybody's sins, and had not yet arose from the dead. His commandments at Matthew 28:18-20, Mark 16:15-16, and Luke 24:45-48 were not made until after His resurrection.
Jesus accepted the thief on the cross while He was still on earth—before He died for anybody's sins. Such a statement from the Lord should come as no surprise to us, because, even before His crucifixion, Jesus Christ had the power on earth to forgive sins. Consider this exchange, which happened well before His death:
MARK 2:2 And straightway many were gathered together, insomuch that there was no room to receive them, no, not so much as about the door: and he preached the word unto them.
MARK 2:3 And they come unto him, bringing one sick of the palsy, which was borne of four.
MARK 2:4 And when they could not come nigh unto him for the press, they uncovered the roof where he was: and when they had broken it up, they let down the bed wherein the sick of the palsy lay.
MARK 2:5 When Jesus saw their faith, he said unto the sick of the palsy, Son, thy sins be forgiven thee.
MARK 2:6 But there were certain of the scribes sitting there, and reasoning in their hearts,
MARK 2:7 Why doth this man thus speak blasphemies? who can forgive sins but God only?
MARK 2:8 And immediately when Jesus perceived in his spirit that they so reasoned within themselves, he said unto them, Why reason ye these things in your hearts?
MARK 2:9 Whether is it easier to say to the sick of the palsy, Thy sins be forgiven thee; or to say, Arise, and take up thy bed, and walk?
MARK 2:10 But that ye may know that the Son of man hath power on earth to forgive sins, (he saith to the sick of the palsy,)
MARK 2:11 I say unto thee, Arise, and take up thy bed, and go thy way into thine house.
MARK 2:12 And immediately he arose, took up the bed, and went forth before them all; insomuch that they were all amazed, and glorified God, saying, We never saw it on this fashion.
Before He gave life on the cross, Jesus Christ had the power on earth to forgive sins. As He did with the sick of the palsy (Mark 2:3), so too with the thief on the cross—their sins were forgiven at the mouth of Jesus Christ. Jesus came to offer His life for the sins of the world. While He was still on the cross—and that thief with Him—His death had not yet been accomplished.
It was not until after His resurrection, after the third day after His death, that He gave the following commandments:
MATTHEW 28:18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth.
MATTHEW 28:19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:
MATTHEW 28:20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
and,
MARK 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
MARK 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
and,
LUKE 24:44 And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
LUKE 24:45 Then opened he their understanding, that they might understand the scriptures,
LUKE 24:46 And said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day:
LUKE 24:47 And that 1.) repentance and 2.) remission of sins should be preached in his name among all nations, beginning at Jerusalem.
LUKE 24:48 And ye are witnesses of these things.
and it was these precise instructions that were carried out by the Apostles, beginning at Jerusalem:
ACTS 2:38 Then Peter said unto them, 1.) Repent, and 2.) be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.