One of the most common arguments against baptism being for the forgiveness of sins is the thief on the cross (Luke 23:39–43). Simply put, the argument goes as follows: since the thief on the cross was forgiven without being baptized, then baptism must not be for the forgiveness of sins. This argument, however, does not hold. First, we’re assuming that the thief was not baptized under John’s baptism – an assumption I’m happy to grant, but still an assumption we need to acknowledge. Even if he was, John’s baptism is not New Testament baptism (Acts 19:1–5).
Second, and more importantly, the thief is still under the Old Covenant, but we are under the New Covenant. There is a fundamental difference between life before Jesus’s death and resurrection and life after those events. It is only after Jesus’s resurrection that Christians are given the command to baptize in the name of the Father, Son, and Holy Spirit (Matthew 28:18–20). It is also under the New Covenant that we are told the purposes of baptism, one of which is “for the forgiveness of your sins” (Acts 2:38). We cannot allow assumptions and inferences based on an Old Covenant event to overrule the clear instruction and explanation given to us under the New Covenant.
Third, the thief on the cross is in an exceptionally unique situation: he is one of only two people who will ever be hanging on a cross alongside Jesus. Unique circumstances cannot be made normative. We all understand this, even those who appeal to the thief in their argument against baptism for the forgiveness of sins. After all, no one is advocating that a person must be nailed to a cross next to Jesus for their sins to be forgiven! Also, it’s noteworthy that nowhere in the New Testament is appeal made to the thief on the cross as an answer to any question, including the question of salvation.
Finally, and most importantly, Jesus is Lord and can instruct and forgive as He sees fit. If He looks at a man and says “be healed,” know that man is healed (e.g., Mark 10:51–52). Or, if He says “go wash, then you will be healed,” then know that once the man does as instructed, he will be healed (e.g., John 9:6–7). Likewise, when Jesus tells the thief on the cross, “today you will be with me in paradise,” we can know for certain that Jesus has forgiven that thief of his sins. But what does Jesus say to us? As mentioned above, He instructs us to be baptized, and part of the explanation as to why is that our sins are forgiven at baptism.
Summing it up: we cannot ignore what Jesus and His apostles teach us plainly by appealing to what Jesus said to someone else in an incredibly unique situation and under a different covenant.
Studies in the Bible 