Several different denominations teach that water baptism is a prerequisite for (or is essential for) salvation. What is the biblical basis against this teaching?
One of the best articles on this controversial subject is from the Christian Apologetics and Research Ministry, entitled Is Baptism Necessary for Salvation? by Matt Slick. I've included some of the key verses it refers to, and arguments it uses, below.
Romans 5:1 and Ephesians 2:8 show that we are justified by faith (not by faith + something else). Furthermore, Romans 4:1-11 talks of being saved by faith, not by works - including the "works" of the Jewish religious ceremony of circumcision. In particular, verses 9b-10:
In other words, Abraham's faith saved him before his circumcision; salvation comes before the religious ceremony, not as a result of it.
As a slight aside, another oft-cited example is the account of Jesus' crucifixion (Luke 23:40-43) where one of the criminals being crucified alongside Jesus is apparently saved without being baptised. Slick does not quote this, but does make mention of the many people who receive Christ on a hospital deathbed and don't get baptised before they die.
Colossians 2:11-12 links the Old Testament ceremony of circumcision with the New Testament ceremony of baptism; therefore what applied to circumcision (it follows salvation, which is by faith) applies to baptism.
1 Corinthians 15:1-5 says that it is by the Gospel that we are saved, and defines the Gospel as Christ dying for our sins and rising again. We are saved by Christ's endeavour, not our own. Paul emphasizes this earlier in 1 Corinthians 1:17a: "For Christ did not send me to baptize, but to preach the gospel" - clearly the two are different things, and it is the latter that Paul sees as the more important.
There are several other passages that say "if X then you are saved" where X does not include baptism; Romans 10:9 is a good one:
Equally, Romans 6:23 describes eternal life in Christ as a free gift - in other words, we do not have to pay, or undergo any religious rite, in order to receive it.
Slick's article then goes on to explain those verses that may seem to say that baptism is part of salvation. That's beyond the scope of this question but I would recommend reading it when looking at this issue, particularly when considering the flip-side of this coin, the question asking for the Biblical basis for baptism as a prerequisite for salvation.
There are a number of compelling arguments against baptismal regeneration, such as
These are dealt with nicely in this answer and in Waggers' answer to this question.
Notice from the passage that
(1) The listeners had not been baptized.
(2) The Holy Spirit fell upon the listeners as Peter was still speaking. Not during baptism.
(3) The circumsized believers were amazed that the Holy Spirit had been poured out on the Gentiles also. Some proponents of baptism-for-salvation claim that the Holy Spirit came upon the Gentiles in a special manner, not indicative of true salvation. This seems unlikely, considering the presence of spiritual gifts (speaking in tongues), and the use of the word "also".
(4) The circumcised believers were amazed that the Gentiles were able to be saved, because they were Gentiles (it is more clear, if you read the context around this passage, that this is the source of their amazement). However, if baptism was the means of salvation, wouldn't the circumcised believers have been equally amazed that the Gentiles were saved without baptism? Yet there is no mention of this incident being an anomaly with respect to baptism (it is an anomaly only because they were Gentiles).
(5) Peter seems to consider receiving the Holy Spirit a prerequisite for baptism. If Peter had believed that water baptism was the means of salvation, why would he see a need for their baptism? It seems to me that he would think: "Wow, what an interesting exception - these people were saved without even being baptized. Well, no sense baptizing them now. They are the saved, after all!" But instead, he instructed them to be baptized. This indicates that Peter saw baptism as a command for believers, and not as a means of salvation.
Baptism is not a part of our salvation, but it is an important command that, as believers, we should follow. It symbolizes the inward change that has already taken place in our lives: our death to ourselves, and resurrection in Christ. (Romans 6:3-4 NASB)