Theologian Alister McGrath outlines the following three stages of receiving what Christ did for us on the cross:

[First], I may believe that God is promising me forgiveness of sins; [second], I may trust that promise; but [third] unless I respond to that promise, I shall not obtain forgiveness. The first two stages of faith prepare the way for the third, without it they are incomplete.

Then he illustrates these three stages with the following true story:

Consider a bottle of penicillin, the famous antibiotic identified by Alexander Fleming, and first produced for clinical use in [Great Britain]. The drug was responsible for saving the lives of countless individuals who would otherwise have died from various forms of blood poisoning. Think of the three stages of faith like this. I may accept that the bottle exists. I may trust in its ability to cure blood poisoning. But nothing will change unless I receive the drug which it contains. I must allow it to destroy the bacteria which are slowly killing me. Otherwise, I have not benefited from my faith in it.

It is the third element of faith which is of vital importance in making sense of the cross. Just as faith links a bottle of penicillin to the cure of blood poisoning, so faith forges a link between the cross and resurrection of Jesus Christ and ourselves. Faith unites us with the risen Christ, and makes available to us everything he gained through his obedience and resurrection.