The problem with TMWWT for modern audiences is that what starts out as
an audaciously plotted thriller takes a sharp turn into religious
parable at the end. Current taste doesn't allow for that. Many people
identify strongly with irreligion nowadays, and there's no question
that some of them would feel angry and betrayed when they got to the
God bits at the end.

If I recommended the book to someone, it would be because I wanted it
to give them pleasure, so my recommendation would be spoiled if it
turned sour for them. The end of a suspense novel is a particularly
vexing place for that to happen. On the other hand, I can't warn
them about it without giving the plot away, which would spoil the
recommendation in a different sense. Hence my dilemma: a risk of
spoilage either way!

In the case of friends I solve this problem by knowing my friend well
enough to recommend the book or not. Not an option on the internet.

I suppose you can say that the end is telegraphed anyhow, but that's
not obvious to every reader. It wasn't obvious to me when I read it--
at least not in the first few chapters, which read like a James Bond
movie 50 years ahead of time and are a large part of what makes the
book so fun.

By the way, on the point about Sunday turning out to be God at the
end, I realize Chesterton repudiated this view
(http://www.cse.dmu.ac.uk/~mward/gkc/books/thursday-after.txt). But
what's striking about that afterword is how feeble it is. I don't find
it convincing in the least. I think Chesterton was covering for how
his artistic imagination carried him beyond the bounds of the doctrine
he subscribed to. That's a sign of good art, in fact.