"Pastor, what do you say to the person who says, 'I trusted Christ for the forgiveness of my sins before I was baptized, so baptism didn't save me'?"
This was his answer in a nutshell.
We do not say that he wasn't already saved. God works through means. Such a person heard the word and received faith as a gift from God by that means. This is not a faith that despises baptism. Even if he does not rightly understand what baptism is and what it does, he still receives it. In his baptism he receives all the gifts that God gives in the washing of regeneration. For this person baptism becomes like a boundary marker. It is a concrete moment in time and space when God was at work to make him a new creature. This is not to despise or forget what God did beforehand, but to give him something unassailable to hold onto in times of trial and temptation.
In the life of every Christian the devil comes to assault us. We know our weakness, our sinfulness, our own unstable, uncertain, wavering and doubtful heart. The devil comes to plow that ground. "Have you given everything to Christ? Are you sure you weren't holding something back? You know how incredible Jesus' story is... Wasn't there something in it that you couldn't quite believe?"
The person who has no foundation for assurance apart from the awareness of his own faith is vulnerable to such an assault. The one who rightly understands what the Bible teaches about baptism can rest in the font. He can say to Satan, "Yes, I am weak and wavering. I am an inconstant friend. Jesus is not. Jesus washed me. Jesus cleansed me from all my sin. I know it for a fact because he came to me on a certain date at a specific place and spoke this promise audibly into my ears. He applied that life giving water to my corrupted flesh. On that day he raised me from the dead, and now I know that I will never die."
Baptism equips the believer for his war against the devil.