Biblical basis for baptism as a prerequisite for salvation Some people believe that if you have not been baptized, you cannot be saved.

Others (including myself) believe that it is merely the outward declaration of what has already happened in the heart.

Given that Jesus baptized no one, and that one of the most famous conversions involved no baptism (the thief on the cross), what Biblical basis does the argument for baptism giving or being a prerequisite to salvation have?
I second the answer of @mason-wheeler that Acts 2:37-38 represents the basic Christian assumption that repentance and baptism, and indeed the entire Christian life that is "the apostles' doctrine and fellowship, and breaking of bread, and prayers," is critical to salvation, and pursuit of salvation outside this life is perilous. Furthermore, resisting a known means of salvation is foolishness and approaching the mindset of the damned. If you are asking whether baptism is required of you, then the answer is almost certainly an emphatic yes, unless you are severely ill or have some other similarly extenuating circumstance such that no one will consent to baptize you. 

If, however, we ask whether baptism is required of our neighbors for their salvation, then we must examine our hearts, for then we are prying into their business.

If, instead, this is a more abstract question, then the answer is that God saves Whom He will save, and we know that He is just. This categorically does not mean that if you die outside of this life, specifically outside of traditional baptism, then you are not favored by God, or that then you are eternally consigned to hell. For Abraham was surely among those raised by Christ in His resurrection, and he was not baptized. Noah was not even circumcised. And surely Christ would not leave his "very good" creations Adam and Eve to languish in the grave! 

It is instructive to consider the case of the thief on Christ's right hand. For even if this thief were not baptized by water and the Spirit, he certainly was baptized with the baptism with which Christ was baptized (Mark 10:38). Many seemingly unbaptized and uncircumcised-of-heart will certainly enter the kingdom of heaven ahead of many of us who are baptized and sell this birthright for bodily pleasure or, worse, spiritual delusion. There are last which shall be first, and there are first which shall be last. (Luke 13:30). (By the way, the thief too is considered a saint of the Orthodox Church, and is celebrated in hymnody as the first man to come back to paradise. The last thing he stole was salvation!)

It is not baptism that saves, but God Who saves. In His wisdom, God has appointed baptism, and anointing with oil, and communion, and marriage, and countless other wonderful mysteries to be effective tools in His inscrutable working-out of our salvation.