Apologies, but I make my debut into this thread that I've been lurking (and why not, it's an intriguing title, but not yet quite got to the end because of the way tracking un-contributed threads works) with a probably re-derail, but...
Anyway, since we're on the subject, "Small Gods" is... religion themed, and makes for a good introduction into the Discworld books.
YMMV, since I tend to recommend a variety of starer-books, depending upon the interests of the person. (If I think they can survive it, I'll at least encourage them to try out TCOM/TLF, with the promise that "this prototype idea grows into something beautifully thought out". Even PTerry doesn't think they're a particularly good start, in hindsight although I think he was happy with the recent adaptation (which, amongst other things, conflated Hrun and Cohen.) But Small Gods has the advantage of being very standalone. Only uses the Discworld concept insofar as a ready base (although his Gaiman novel and Nation were Roundworld-based, so not sure how necessary that is) and gives an interesting inversion to the "everybody knows/believes" idea.
He did also do some scifi books, but I've always felt they're a little "meh".
Dark Side Of The Sun and of course the one with the proto-discworld (tech-based, masquarading as magic, instead of the inverse) were early creations too. They'd be different these days. And the Truckers Trilogy is a children's series from his early days. As, indeed was TCP, albeit written when he was... 16? and revised when he was older... Original "just the teenage author" versions are valuable for their rarity, though not necessarily for their writing style.
Look to his more recent "children's" books (I consider them mainstream canon, plus TAMAHER, though YMMV), and the way the Tiffany Aching series has led. Dark. Approaching the darkness of Thud, in places, with the I Shall Wear Midnight one. We're talking teenage pregnancy and... more. Which leads us, quite unexpectedly, back on topic.
It would be too broad a church to say that I agree with everything Pratchett expounds, but when it comes to some of the key issues in this thread, I think I agree with the way he portrays things. (Ok, so not literally in the case of the gods, but certainly the lifecycle of any given god 'meme' and, on the whole, I'd prefer the practicality of the Witches (ranging all the way from Granny to Tiffany) and their communities to the way many Roundworld communities dictate things, when it comes to this thread's subject.
Anyway, I should stop myself before this turns into an essay on why I like Terry Pratchett.
Ditto.