Some believe that the story of the dying thief (Luke 23:39-43) shows how to become a Christian, and in particular, that one does not need to be baptized to be saved. The idea is that the thief was saved without being baptized, so why shouldn't other people be?
John had baptized multitudes. "All the people were going out to be baptized by him" (Mark 1:4-5). The disciples of Jesus were reported to be baptizing even more disciples than John (John 4:1-2). Many of these disciples later fell away (John 6:66).
Nobody can say with any certainty that the thief on the cross was unbaptized. He is therefore not a definite case of salvation without baptism.
Since there is no confirmation in scripture about the thief being unbaptized, those who use this example as a case for salvation without baptism are basing their doctrine on an assumption from silence. The passage is silent about whether or not the thief had been baptized, so it is assumed that he wasn't. But silence is not proof.
Listen to the word God has spoken rather than to what you imagine you hear in God’s silence. The word of God is not cancelled by the silence of God.
Those of us who believe baptism is necessary for salvation make no assumptions based on silence of the scriptures. Rather, we go to the numerous passages that speak about baptism. We derive a doctrine of baptism from the word, not from silence. For example...
Jesus made a promise to the thief, "Today you shall be with me in Paradise" (Luke 23:43). Jesus was still alive to utter this promise and grant salvation to the thief. The thief was saved when Christ made this pronouncement.
The thief was therefore among the last persons to be saved under the old covenant. He was not among the first to be saved under the new covenant. We know this because "a covenant is not in force while the one who made it lives" (Hebrews 9:16-17).
The "blood of the covenant" (Mark 14:24) had not yet been shed. The death, burial, resurrection, and ascension of Jesus had not yet occurred. The baptism of the new covenant had not yet come into effect. The thief's salvation and assurance of Paradise is therefore not an example of new testament salvation.
If we want examples of how people were saved under the new covenant, when Christian baptism applied, then we should go to examples that took place after the death, burial, resurrection, and ascension of Jesus. The promise Peter made to the People on Pentecost (Acts 2:37-39) is such an example. So is the conversion of Saul (Acts 22:16).
The thief left it till his last hours of life to be saved. Is this a good example of how people ought to be saved? This example of someone being saved as he draws his dying breaths, is certainly very touching. It is a beautiful example of the mercies of Christ.
However, the thief on the cross is most unsuitable and inapplicable as an example of what people should do to be saved. This is obvious. We would not tell people to be like the dying thief and wait till they are almost dead to be saved.
Since the dying thief is an exceptional case, his case should not be raised as an example for the salvation of people in general. There are far better examples, such as the case of the Ethiopian eunuch, the Philippian jailor, or Lydia the seller of purple (Acts 8:38 16:15,33).