Is baptism necessary for salvation?

No. Let's examine what the Scriptures teach on this issue:

First, it is quite clear from such passages as Acts 15 and Romans 4 that no external act is necessary for salvation. Salvation is by divine grace through faith alone (Romans 3:22, 24, 25, 26, 28, 30; 4:5; Galatians 2:16; Ephesians 2:8-9; Philippians 3:9, etc.).

If baptism were necessary for salvation, we would expect to find it stressed whenever the gospel is presented in Scripture. That is not the case, however. Peter mentioned baptism in his sermon on the day of Pentecost (Acts 2:38). However, in his sermon from Solomon's portico in the Temple (Acts 3:12-26), Peter makes no reference to baptism, but links forgiveness of sin to repentance (3:19). If baptism is necessary for the forgiveness of sin, why didn't Peter say so in Acts 3?

Paul never made baptism any part of his gospel presentations. In 1 Corinthians 15:1-4, Paul gives a concise summary of the gospel message he preached. There is no mention of baptism. In 1 Corinthians 1:17, Paul states that "Christ did not send me to baptize, but to preach the gospel," thus clearly differentiating the gospel from baptism. That is difficult to understand if baptism is necessary for salvation. If baptism were part of the gospel itself, necessary for salvation, what good would it have done Paul to preach the gospel, but not baptize? No one would have been saved. Paul clearly understood baptism to be separate from the gospel, and hence in no way efficacious for salvation.

Perhaps the most convincing refutation of the view that baptism is necessary for salvation are those who were saved apart from baptism. We have no record of the apostles' being baptized, yet Jesus pronounced them clean of their sins (John 15:3--note that the Word of God, not baptism, is what cleansed them). The penitent woman (Luke 7:37-50), the paralytic man (Matthew 9:2), and the publican (Luke 18:13-14) also experienced forgiveness of sins apart from baptism.

The Bible also gives us an example of people who were saved before being baptized. In Acts 10:44-48, Cornelius and those with him were converted through Peter's message. That they were saved before being baptized is evident from their reception of the Holy Spirit (v. 44) and the gifts of the Spirit (v. 46) before their baptism. Indeed, it is the fact that they had received the Holy Spirit (and hence were saved) that led Peter to baptize them (cf. v. 47).

One of the basic principles of biblical interpretation is the analogia scriptura, the analogy of Scripture. In other words, we must compare Scripture with Scripture in order to understand its full and proper sense. And since the Bible doesn't contradict itself, any interpretation of a specific passage that contradicts the general teaching of the Bible is to be rejected. Since the general teaching of the Bible is, as we have seen, that baptism and other forms of ritual are not necessary for salvation, no individual passage could teach otherwise. Thus we must look for interpretations of those passages that will be in harmony with the general teaching of Scripture. With that in mind, let's look briefly at some passages that appear to teach that baptism is required for salvation.

In Acts 2:38, Peter appears to link forgiveness of sins to baptism. But there are at least two plausible interpretations of this verse that do not connect forgiveness of sin with baptism. It is possible to translate the Greek preposition eis "because of," or "on the basis of," instead of "for." It is used in that sense in Matthew 3:11; 12:41; and Luke 11:32. It is also possible to take the clause "and let each of you be baptized in the name of Jesus Christ" as parenthetical. Support for that interpretation comes from that fact that "repent" and "your" are plural, while "be baptized" is singular, thus setting it off from the rest of the sentence. If that interpretation is correct, the verse would read "Repent (and let each of you be baptized in the name of Jesus Christ) for the forgiveness of your sins." Forgiveness is thus connected with repentance, not baptism, in keeping with the consistent teaching of the New Testament (cf. Luke 24:47; John 3:18; Acts 5:31; 10:43; 13:38; 26:18; Ephesians 5:26).

Mark 16:16, a verse often quoted to prove baptism is necessary for salvation, is actually a proof of the opposite. Notice that the basis for condemnation in that verse is not the failure to be baptized, but only the failure to believe. Baptism is mentioned in the first part of the verse because it was the outward symbol that always accompanied the inward belief. I might also mention that many textual scholars think it unlikely that vv. 9-20 are an authentic part of Mark's gospel. We can't discuss here all the textual evidence that has caused many New Testament scholars to reject the passage. But you can find a thorough discussion in Bruce Metzger, et al., A Textual Commentary on the Greek New Testament, pp. 122-128, and William Hendriksen, The Gospel of Mark, pp. 682-687.

Water baptism does not seem to be what Peter has in view in 1 Peter 3:21. The English word "baptism" is simply a transliteration of the Greek word baptizo, which means "to immerse." Baptizo does not always refer to water baptism in the New Testament (cf. Matthew 3:11; Mark 1:8; 7:4; 10:38-39; Luke 3:16; 11:38; 12:50; John 1:33; Acts 1:5; 11:16; 1 Corinthians 10:2; 12:13). Peter is not talking about immersion in water, as the phrase "not the removal of dirt from the flesh" indicates. He is referring to immersion in Christ's death and resurrection through "an appeal to God for a good conscience," or repentance.

I also do not believe water baptism is in view in Romans 6 or Galatians 3. I see in those passages a reference to the baptism in the Holy Spirit (cf. 1 Corinthians 12:13). For a detailed exposition of those passages, I refer you to my commentaries on Galatians and Romans, or the tapes of my sermons on Galatians 3 and Romans 6.

In Acts 22:16, Paul recounts the words of Ananias to him following his experience on the Damascus road: "Arise, and be baptized, and wash away your sins, calling on His name." It is best to connect the phrase "wash away your sins" with "calling on His name." If we connect it with "be baptized," the Greek participle epikalesamenos ("calling") would have no antecedent. Paul's sins were washed away not by baptism, but by calling on His name.

Baptism is certainly important, and required of every believer. However, the New Testament does not teach that baptism is necessary for salvation.  http://www.gty.org/Curiosity_Shop/baptism.htm

But what of those verses that seem to teach faith plus baptism as a means of salvation?  Mk 16:16 states, "whoever believes and is baptized will be saved, but whoever does not believe will be condemned."  The NT certainly assumes that every believer will be baptized, and this is reflected in the first half of 16:16.  However, we should note that according to the second half of the verse, condemnation comes as a result of unbelief, not the lack of any ritual activity (e.g., baptism).  Taken in isolation someone might possibly misunderstand 16:16 as teaching baptismal regeneration, but, when compared with the rest of Scripture, this misunderstanding evaporates.  In any event, "whoever does not believe will be condemned" puts the emphasis on faith, not baptism. 

Perhaps the most popular text of baptismal regenerationists is Ac 2:38, "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins."  The crux of Peter's intended meaning lies in the word "for."  This one little preposition (eis) is translated forty-seven different ways in the NASB; it would be unwise to build a theology of baptismal regeneration on a single word with such a broad range of meanings!  Even if one wants to maintain that eis here connotes a goal, it still does not follow that baptism is necessarily involved.  This can be shown from a grammatical standpoint in the Greek text.  The phrase "for the forgiveness of your sins" (lit., "the sins of all of you") agrees in person and number with the command to "repent" ("all of you repent"); both are second-person plural.  The phrase "let each of you be baptized," on the other hand, stands alone grammatically since it is in the third person singular ("let each one").  The word order of the Greek makes little difference; it is the grammatical agreement that matters.  Thus, the text should be translated, "Repent, all of you, for the forgiveness of your sins; and let each one of you be baptized."
 In Ac 22:16, Ananias told Saul, "Get up, be baptized and wash your sins away, calling on His name."  Taken in isolation this too could be taken to teach baptismal regeneration.  There are, however, better alternative explanations for this verse.  It is fully conceivable that the text is to be translated, "be baptized, and wash away your sins by calling on his name," hence connecting "washing" with "calling" and not with "baptism" (which merely symbolizes the washing effected by calling on his name).  Alternately, Ananias may simply be speaking metaphorically of that which baptism symbolizes-the washing away of sins.
 Another exegetical blunder is to read water baptism into Ro 6:1-10. Baptizo simply means "immersion"; the element into which that occurs must be observed from context.  There are several kinds of immersion in the NT, including immersion into the Holy Spirit (Jn 1:33), fire (Mt 3:11), Christ (Ga 3:27), Moses (1 Co 10:2), and, of course, water.  Whereas in English the word "baptize" has exclusively religious connotations, it was not so with the Greek word baptizo (which evoked as much religious imagery as the words "dip" or "plunge under" would today).  Thus, it is a mistake to read the word "baptize" in the Bible and always think of a religious ritual involving water.  Ro 6 speaks of immersion into Christ Jesus and his death, which occurs by grace through faith (Ro 5:17).  The word  "water" is completely absent.  The same is true of Ga 3:26-29, which refers to being "baptized into Christ" (not water).  It is a spiritual baptism that places us into Christ, not a water baptism.  Literally translated, Ga 3:26-27 states, "you are all sons of God through faith in Christ Jesus, all of you who were immersed into Christ have clothed yourselves with Christ."  Clearly, being immersed into Christ is paralleled with having faith in Christ.
 Yet another example of a non-water immersion is 1 Co 12:13, "For we were all immersed by one Spirit into one body . . . and we were all given the one Spirit to drink."  Here the baptizer is the Spirit and the element into which we are immersed is the body of Christ, not water.  One should no more associate this baptism with water than one would associate the "drink" of 12:13 with water.
 Tit 3:5b ("He saved us through the washing of rebirth and renewal by the Holy Spirit") has been used to justify the necessity of water baptism in order to be saved.  Does the word "water" appear anywhere in the chapter?  Indeed, where is the word "baptism"?  "Rebirth" actually does have a "washing" effect (it washes away our sins), but to read water baptism into this passage is truly to force into it something that is not there.  In fact, 3:5a states, "he saved us, not because of righteous things we have done."  What is water baptism but one of the "righteous things" which we might do?  Verse 7 goes on to reveal clearly that we have been "justified by his grace" (not by water baptism). 

 But what of the seemingly irrefutable verse, "baptism now saves you" (1 Pe 3:21)?  First, note that whatever Peter meant, he did not have water baptism in mind since Peter himself went on to write: "not the removal of dirt from the flesh."  Having just written of Noah's salvation from the flood, Peter was reminded of our salvation from sin.  He thus wrote that there is a "correspondence" (antitupos) between Noah's salvation and our salvation.  The "baptism" Peter referred to was not water baptism, but rather a metaphorical baptism:  "the appeal to God of a good conscience."

If water baptism really were a necessary condition to being forgiven, then baptism would have to be included as a part of the gospel message.  However, regarding baptism Paul wrote, "for Christ did not send me to baptize, but to preach the gospel" (1 Co 1:17).  Prior to this he had written, "I am thankful that I did not baptize any of you except Crispus and Gaius" (v 14).  Do these sound like statements from a man who believed in baptismal regeneration?  Clearly baptism is not a part of the gospel, nor required in order to be saved.

God's people have always and only been saved by grace through faith.  Abraham was justified by believing God (Ge 15:6), and this is the pattern for NT believers as well (Ro 4:9-12, see also Heb 10:4; Heb 11; Lk 7:36-50; Lk 18:13-14; Lk 23:39-43).  By way of balance, it should be pointed out that any person who has experienced God's grace will respond with both faith and a desire to obey his commands.  Thus, every true believer will naturally want to be baptized.  One of the reformers correctly said, "we are saved by faith alone, but a faith that saves is never alone" (it is always accompanied by good works).  Therefore, while baptism is unnecessary for forgiveness, a person claiming to believe, but refusing baptism, is of questionable genuineness.

The error of baptismal regeneration is that it requires man to do something (in this case, be baptized) in order to be saved.  Water baptism is certainly an important result of salvation, but not a means to salvation.  The theology of baptismal regeneration is the result of not truly understanding the gospel of grace.  The perverted "gospel" condemned in Galatia was that of faith in Christ plus circumcision.  The lesson derived from this is that a "gospel" of faith in Christ plus anything is really "no gospel at all" (Ga 1:7). 

http://www.ntrf.org/regen.html