If the atheist is correct, then the Christian has somehow tapped into an unidentified power to become a new creation. At the end of his/her existence, that Christian was able to leave a more positive impact on the world because of his/her faith.
But what if the Christian is right?
Well, not necessarily. It's pretty clear that belief or non-belief is a very poor indicator as to whether or not someone will do positive things in their life.
As evidence of this, I cite the entirety of world history since the death of Christ.