That I brought up The Passion of the Christ in passing yesterday brought up the atonement question. Many of us, and perhaps quite rightly, were uncomfortable with the soteriology expressed by the film. It seemed to many viewers that the doctrine behind the film was that it was the very intensity of Christ's sufferings that saves us. Of course this is a vulgar and inadequate sense of who God is.
So, real quick and dirty, here's my take on the question. We human beings are prone to hurting each other. We're selfish and self-involved. We value our own security at the expense of the lives of others. All of this blossoms into sinful habits, wars, and the unjust structures of society. This is the great and horrible mystery we call original sin. At the heart of it are the mysteries of despair and death.
God loves the world so much that the Word of God, the perfect image of God the Father from all eternity, takes flesh in order to deliver us from this unfortunate situation. He shares in our self-inflicted punishment of despair and death through his Passion and Crucifixion.
But...mysteriously united to the historical human life of Jesus of Nazareth is the divine life of the Eternal Word. Thus the death of Good Friday can't hold the life, both human and divine, of Jesus Christ. The divine life of the Word of God is too strong to be held by the peculiar uncreation we know as death. Passing through our death, the Word of God opens a path for human nature back to blessedness and freedom from sin. Jesus Christ, human and divine, passes through death and establishes a path for us to do the same. This is the mystery we call the Resurrection. "It is the passover of the Lord." (Exodus 12:11)
The path through despair and death is open to us in the Resurrection. This is why we say that our baptism is a baptism into the death of Christ. (Romans 6:3) When our initiation into Christ is completed in the Eucharist, our humanity is mingled with the humanity of Christ, and we come into the very life of the Trinity through adoption.