Once...and for all?

We watched "The Passion of the Christ" (Mel Gibson's film) today...it's a powerful portrayal, to say the least.

The Passion Story has left me with a question this year: "If the death of Jesus on the cross was for the 'sins of the whole world' must one even KNOW about it for it to be effective?"

Isn't the atonement effective because of what Jesus did, regardless of who believes? My faith doesn't add to or subtract from the death of Christ or its power (although my story IS part of his-story). Why should someone else's lack of faith take away from the power of the atonement?