Sometimes when we, members of the church of Christ, say baptism is essential for salvation, a number of people accuse us of believing water actually washes away sins. They get the impression we think the water has some magical property to forgive sin. I Peter 3:21 actually refutes this but people apparently don't hear what we are saying. The context speaks of the salvation Noah enjoyed through the ark and global flood waters, and Peter states that baptism is an antitype or corresponding figure to it. "There is an antitype which now saves us--baptism (not the removal of the filth of the flesh, but the answer of a good conscience toward God), through the resurrection of Jesus Christ." Thus, Peter was not talking about taking a physical bath to wash away dirt, but he does say baptism saves because it accomplishes the spiritual cleansing of the soul. Obviously, there is a close analogy between the two, however. Earlier, in the same letter, Peter reminded his readers that they had "purified [their] souls in obeying the truth" (I Pet. 1:22). Baptism, when preceded by a penitent faith, is the culminating act of obedience that saves. It is that simple.
The spiritual cleansing occurs because in baptism a sinner comes into contact with the blood of Christ. How so? Well, Romans 6:3,4 gives the answer - "Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life." Christ's blood was shed in His death. His blood cleanses us of our sins - "To Him who loved us and washed us from our sins in His own blood" (Rev. 1:5). But, how does Jesus cleanse us with His blood? We can't touch it physically. So, it must be done spiritually. This is what Paul alluded to in Romans 6:3 ("baptized into His death"). Notice that verse 4 says that it is only after being baptized that one arises to walk in newness of life. This shows that before baptism a person is still a lost sinner (cf. Rom. 6:5-7).
Peter had to urge his Jewish brethren to obey on Pentecost - "And with many other words he testified and exhorted them, saying, 'Be saved from this perverse generation'" (Acts 2:40). Peter's statement, "Be saved," proves they were lost even though at that very moment they believed Jesus to be the Son of God. They were not saved before repenting and being baptized.
A sinner comes into contact with the blood of Christ in baptism. From that point forward he continues to have access to the blood of Christ to keep his soul clean. John wrote - "But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin" (I John 1:7). This blessing is available only to those who are in Christ. This is true since all spiritual blessings are found in Christ (Eph. 1:3). But, how does one get into Christ? Hear what the Spirit says - "For you are all sons of God through faith in Christ Jesus. For as many of you as were baptized into Christ have put on Christ" (Gal 3:26,27). Thus, a person is not a son of God (i.e., not a Christian) until he has been baptized into Christ. Why is that? Because until he contacts the blood of Christ he is still in his sins.
The truth found in I Peter 3:21 is simple enough to understand. The cleansing blood of Christ must be contacted through the waters of baptism if one is to be saved. However, when a person has been taught something contrary to this he will find it hard to let go of it. He must believe and obey the truth if he wants to go to heaven.