"The central Christian belief,” C. S. Lewis once wrote, “is that Christ’s death has somehow put us right with God and given us a fresh start. Theories as to how it did this are another matter. A good many theories have been held as to how it works; what all Christians are agreed on is that it does work.” There are still a good many theories, but theologians today are less in agreement that it is Christ’s death that has put us right with God.
Most Christians probably think of a “satisfaction” or “substitution” model of Christ’s work as the standard story, even if they don’t believe that story themselves. Roughly summarized: our sin leaves us owing God a debt we cannot pay; since we owe God everything, we can never catch up once we have fallen behind. But Christ suffered and died for our sins, doing on our behalf what we could not do for ourselves and thereby accomplishing our salvation.