I admire a lot of C.S. Lewis' writing, and even though his pro-Christian message does not persuade me, I can often respect the intelligence of his arguments.
However, I hated MERE CHRISTIANITY. As I recall, it started out as a series of radio broadcasts intended to familiarize the lay Christian with the fundamentals of his belief. Deprived of his natural milieu of academia, I found MC fussy, self-important, and plagued by some horrible gaps in logic. But then, I admire Henri Bergson's thought, and Lewis sophistically dismissed him in a couple of sentences, so that may have prejudiced me.