How do you answer the objection that belief in baptism being necessary for salvation is false because it adds to the already finished work of the cross? For example, one person... states: "Today I maintain that baptism has no part in Salvation. It has to be the finished work of the Cross plus nothing. Otherwise we are telling God, 'You didn’t quite finish it. I have to complete it with my baptism.' So here is where we have to be so careful. When someone questions my stand on baptism, I tell them,'Don’t ever make it part of your Salvation, because then you are adding to the finished work of the Cross. We rest totally on what Christ accomplished on our behalf.'” Any thoughts about this topic would be appreciated. Thanks—Kedron Jones, Apologetics Research Society

I think that makes as much sense as saying that repentance, or confessing Jesus as Lord, adds to the finished work of the Cross.

Now most evangelicals require us to "receive Christ" through the expression of a prayer of faith. But this is doing something. Baptism, on the other hand, is passive—it's something done by others to you. The Sinner's Prayer—it's something you do yourself. Instead of the Sinner's Prayer, the New Testament has baptism. But functionally, according to their proponents, they are similar: their advocates require something.

And there is nothing meritorious about receiving a gift.