While some may over-simplify the concept of “writing the zeitgeist,” there are more possibilities than merely portraying or capturing the spirit of one’s age (simple digestion and regurgitation). Reflecting upon or illustrating the prevailing attitudes (or spirit) of one’s day may well mean contradicting some or all of it, as in, finding fault with it.
C.S. Lewis certainly revealed what he perceived as the flaws of problematic attitudes of his day (although perhaps not “prevailing” as of yet), as he took to task the “bent” mindsets arising then and compelled his contemporary readership into needful contemplation while he corrected what he saw as crucial errors. In so doing, he preserved for modern readers (of many decades later) a glimpse into just what sorts of attitudes and issues were around way back then; the thought processes of that day, the questions, issues, debates, etc., are in various ways answered or addressed or at least mourned as they are somewhat memorialized in this well-written sci-fi. The degree to which Lewis accurately described “the sin problem” (which is persistent in any age of sin-cursed humans) is the degree to which the problems he described (and errors he challenged) transcend his day. In this regard, he nailed it. Thus the book has long outlasted his day’s zeitgeist.
The “science” of this fiction is in some ways so antiquated it seems absurd to modern readers, yet the skillful writing, wonderful wisdom, and timeless wit serve to make some now-inane sci-fi premises (e.g. of Mars, in book #1, and Venus, in book #2, being habitable places with atmospheres hospitable for humans) into mere trifles worth overlooking and easily overlooked. Outdated? In certain aspects, yes. Still worth reading? Absolutely.