Miracles is a prescriptive document, and a narrow one at that. It is brilliantly argued, which is both its greatest strength and fatal weakness. The excerpts I've posted are part of a largely, internally, consistent and original way of thinking about a subject as old as thought itself.
But as Lewis's ideas are so particular, they brook absolutely no compromise or alternative, leading to a gross lack of self-awareness that makes them endlessly frustrating to an outsider. Lewis devotes a whole chapter, "Christianity and 'Religion'" to separating his idea of Christianity from the rest of the "Pantheistic" belief systems. He uses the testimony and martyrdom of the early Christians as proof of Jesus's historicity and divinity, while not bothering at all to bring up the similar convictions of the original Mormons. He dismisses other purported miracles while saying that "we seldom find the Christian miracles denied except by those who have abandoned some part of the Christian doctrine." It takes considerable audacity to remove Christianity from the realm of religion, and then to feign humility by calling it "mere."
Yet even this intolerance has its upside. For Lewis is almost as contemptuous of popular Christianity as materialism, leading him to write something like this:
Many a man, brought up in the glib profession of some shallow form of Christianity, who comes through reading Astronomy to realise for the first time how majestically indifferent most reality is to man, and who perhaps abandons his religion on that account, may at that moment be having his first genuinely religious experience.
That allowance of possibility, as opposed to a clearly and logically articulated single path, is most appealing, and while not ultimately a large part of Lewis's thought, it greatly informs The Screwtape Letters, if only because it is not making a positive argument but, through its sharp irony, identifying the undesirable.
The book is filled with countless barbs that sting because they are so true.
In civilised life domestic hatred usually expresses itself by saying things which would appear quite harmless on paper (the words are not offensive) but in such a voice, or at such a moment, that they are not far short of a blow in the face.
All mortals end to turn into the thing they are pretending to be.
The safest road to Hell is the gradual one--the gentle slope, soft underfoot, without sudden turnings, without milestones, without signposts.
The relationship itself between Screwtape and his "dear" Wormwood provokes an especial revulsion, with its airs of tenderness masking a wholly power-driven and literally consumptive parasitism:
My dear, my very dear, Wormwood, my poppet, my pigsnie,
How mistakenly now that all is lost you come whimpering to ask me whether the terms of affection in which I address you meant nothing from the beginning. Far from it! Rest assured, my love for you and your love for me are as like as two peas. I have always desired you, as you (pitiful fool) desired me. The difference is that I am the stronger. I think they will give you to me now; or a bit of you. Love you? Why, yes. As dainty a morsel as ever I grew fat on....
....Your increasingly and ravenously affectionate uncle,
These are resonant because they are easily recognizable in one's everyday affairs. The shudder one feels--should feel!--when reading it is a call for vigilance in one's own life, lest one should find oneself falling into such an arid, often unconscious, mode of thought.
This is a more useful, more universal, application of Lewis's dire warnings at the end of Miracles, and doubtless one Lewis would disagree with vehemently. But I daresay no thinker or philosophy should ever be accepted wholesale, and Lewis is no different--to his credit. I find C.S. Lewis inscrutable, reactionary, and often wrong, but he is also eminently well-written and capable of great insight. This much can be said about Christianity, too, which Lewis would likely agree is the highest praise I could offer.