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.
One wonders why it took Lewis so many words to say, "This is what you should believe because I say so. Don't expect me to explain it because I've no more idea than fly how it works, or even what it does. Just believe what I say, or my imaginary friend will get you. Okay!"
His arrogance would be astounding if we didn't know that in deferential Britain of 1952, people from his class were expected to be arrogant. They believed they held a monopoly on wisdom and their burden was to dispense it to the rest of us lesser beings. Expecting them to explain their reasoning to us would have been getting above ourselves. It was enough that they believed it themselves. Why would they need to bother with reasons when this knowledge just popped itself into their heads, ready to be dispensed? Simple oiks like us needed to be told what to believe otherwise there would be anarchy. Lewis was merely shouldering his burden - and getting lots of money for it.