With an ‘s’ or a ‘k’ sound? I’ve been wondering about this for a long time and got two reference points: the famous soccer club Celtic Glasgow, which I’ve never heard otherwise than as “sseltic”, and the Van Morrison song “Celtic Ray” where he sings “keltic”. Is it a dialect/regional thing?
Well for me it’s a usage thing: if it’s a basketball team from the NE then it’s Sell-tick. Anything else and it’s Kell-tik.
https://www.digitalmedievalist.com/opinionated-celtic-faqs/keltic-or-seltic/
They’re both valid, it’s all about context and history.
Wow well, I thought that this was just a mundane question from a non-native speaker, but I’d never thought that this is a hot topic in some circles! How the dope never fails to surprise (or the English language, for that matter).
Keltic unless it’s the Boston NBA team, then Seltic.
I have the same confusion with other words from Latin. I know that ancient Latin used the “K” sound for all written "C"s. Caesar was pronounced “Kaisar”. But you know, that was 2000 years ago, and languages change. In particular, the family of French/Italian/Spanish/Romanian is basically modern Latin, and they pronounce "C"s as "S"s all the time. So I tend to pronounce “Caesar” as “Seezer” in deference to hundreds of years of English (and Romance) tradition, but other less common Latin words trip me up sometimes. Like “Cicero”.
Relevant Wiki article.
The earliest attested form is from the Greek, and the word is “Κελτοί” (“keltoi”). From about 500 BC, speaking of (what we would later call) Gauls in (what we would later call) France and Germany. That carried through with the “hard K” customary pronunciation of “C” in Classical Latin. Later Latin is where that leading “C” became palatalized, shifting into the “S” sound we use a lot now.
Interestingly, those early-historical roots of the word “Celt” were only applied to continental Europeans Gallic peoples, not the Celts of the isles; this is rather different from modern usage, which usually would call the former “Gauls” and use “Celts” for the latter.
Both are technically correct. Though I’m not a prescriptivist, if you use the “s” sound for anything but the Boston basketball team **or **the Glasgow association football team, then your knowledge in the subject is suspect.
Ave, true to Kaiser. Degenerates like you belong on a cross.
When I first heard of the Keltic people, back in the Eisenhower Administration, the books I read spelled them as Kelts.
Wouldn’t that spelling help us all get along?
Maybe, but the reason is that there is no “k” in Irish, Scottish, or Welsh, the dominant 3 Celtic languages. Cornish, Breton, and Manx do have it, but I think it’s always hard.
Kelts sounds like some street gang from a cyberpunk novel.
Or like small, thick, multi-purpose mat/coaster/rug/towel things. The kind that are so useful that they’re too often dirty when you need one.
Hmmmm. My Scottish friend John is like that - small, thick, very useful, too often dirty when you need him. 
And I just realized he IS quite absorbent as well.
But that misses the example of Glasgow Celtic, the football club. Which is always pronounced with the “soft” version in Scotland, regardless of allegiance - as indicated in the OP.
Which in turn has made the pronunciation of the term for the people/language/culture/made-up-historical-construct often a bit delicate in Scotland.
There’s a story about Richard Burton, who was apparently approached at a party by an Irish-American who said he too was a ‘Selt’. Burton replied to the effect “I am a Celt, You sir, are a Sunt”
(probably apocryphal).
Bit like that across the water in N Ireland I think.
See also the pronunciation of the name of the Mater hospital in Belfast. The influence of the latin mass allegedly shows up Catholics who pronounce it correctly.