The central Christian belief 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 different theories have been held as to how it works; what all Christians are agreed on is that it does work. I will tell you what I think it is like…. A man can eat his dinner without understanding exactly how food nourishes him. A man can accept what Christ has done without knowing how it works: indeed, he certainly would not know how it works until he has accepted it.
We are told that Christ was killed for us, that His death has washed out our sins, and that by dying He disabled death itself. That is the formula. That is Christianity. That is what has to be believed. Any theories we build up as to how Christ’s death did all this are, in my view, quite secondary: mere plans or diagrams to be left alone if they do not help us, and, even if they do help us, not to be confused with the thing itself. All the same, some of these theories are worth looking at.
C.S. Lewis, Mere Christianity (1952; Harper Collins: 2001) 54-56.