Probably the best place to go to talk about the relationship between (water) baptism and salvation is 1 Peter 3:21-22:
21 Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a good conscience, through the resurrection of Jesus Christ, 22 who has gone into heaven and is at the right hand of God, with angels, authorities, and powers having been subjected to him.
The point Peter is making about water baptism is that it is a sign of a greater reality that actually saves you: the appeal to God for a good conscience. That is, I am not saved by being baptised. But baptism is a sign, a symbol, of something that does save me: Jesus’ death and resurrection, which I appropriate to myself by asking God to cleanse my conscience.
Therefore, water baptism in the New Testament was the most common way that people were saved – not because the baptism itself did anything for them, but because in undergoing the sign of baptism they were asking God to save them. And in our day too, water baptism is still a very appropriate and helpful sign (or, to use a more old-fashioned term, a “sacrament”) of this reality.
The problem comes, though, if you start confusing the sign with the reality. Take a more modern example: a road sign. Let’s say you ask me for directions to the St Michael’s Night Church Getaway next year (27-29 April 2007, in case you were wondering). I tell you, “Go south along the Princes Highway, and when you see the sign that points to Camp Koloona, follow that sign and you’ll get to the camp site”. Now just suppose that I’m sitting there at the camp site and you turn up successfully. I say to you, “Good to see you at the Night Church getaway! You must have followed the sign! It was a good sign, wasn’t it? Big, bold letters, nice colours.” And you tell me, “Actually, no, I didn’t see the sign, and I had to drive a few extra km out of the way, but I got here anyway”. How should I react? I should be glad, of course, that you got to the camp site! But what if I said to you, “Well, mate, I’m sorry, but in fact you’re not actually at Camp Koloona. The way to get to Koloona is to follow the sign, and you didn’t follow the sign. Therefore you’re not here. Sorry, but I can’t talk to you because you’re not actually here. La la la la la…” That would, of course, be crazy.
But it’s the way people can sometimes talk about baptism (“You’re not baptised? Sorry, but you’re not saved!”). Baptism is a sign of salvation – but people can be saved without undergoing the sign (take, for example, the criminal on the cross next to Jesus in Luke 23:42-43). The may have missed out on the sign, but if somebody has appealed to God for a good conscience through the resurrection of Jesus from the dead, then they are saved. It may still be a good idea to undergo the sign of baptism – since baptism is a good, appropriate sign of salvation. But what is wrong is to confuse the sign with the reality and insist that a person must be baptised with water otherwise they can’t possibly be saved.