Apparently, we can blame the priesthood for this abomination.
In Vulgate Latin, the letter ‘c’ when followed by an ‘i’ or an ‘e’ was pronounced as a soft ‘c’ (or ‘s’). This (along with the prohibition of ending a sentence with a preposition, among other anachronistic silliness) carried over into Norman French and then into English.
The pronunciation changed. I don’t think there’s anything else to it. It was originally pronounced Keltic, got shifted to Seltic, and more recently (meaning after the team started playing) went back to Keltic.
Spelling is incidental; it actually goes back to pre-writing pronunciation, meaning, and intent. The prefixual pronunciation kell originally implied a sort of gift or dowry. This, of course, was not appropriate for the general purpose of starting a basketball club, which though premised on a sport, is really a business venture. Therefore, the decision was made to use a more relevant prefix pronunciation with its different applied meaning: sell-tix.
Rod Stewart’s song “You’re in My Heart” is an extended series of metaphors describing the beloved as “ageless, timeless, lace and fineness, beauty and elegance,” among other things. Then in an extended play on infatuation and sports-fandom, he sings:
“You’re an essay in glamour
Please pardon the grammar
But you’re every schoolboy’s dream
You’re Celtic, United, but baby i’ve decided
You’re the best team i’ve ever seen”
Celtic (pronounced with the initial /s/) and United are of course the two competing Glaswegian teams.
It’s not a ridiculous prescriptive rule like the preposition one; it’s a simple, standard pronunciation pattern. It goes for the letter c before y, as well, though that doesn’t come up as often. This is why ‘accident’ is pronounced as if it has an x in the middle - the first c, which is not before e, i or y, is a hard c, and the second c, which is before i, is a soft c.
So people saying seltic for those two sports clubs were saying it like an English word, not a, er, Celtic word, and that pronunciation stuck.
Spanish and French have a similar spelling/pronunciation pattern for the letter c.
A few years ago “All Things Considered” did an April 1 story about the Boston Celtics signing a hot Irish prospect, who insisted on a clause that the organization must change its pronunciation from Seltic to Keltic.
Gaulish pronunciation: /k/ (long since defunct)
The modern languages are also all /k/, but learned borrowings rather than cognates.
Classical Latin pronunciation: /k/ (“Keltic”)
Church Latin pronunciation: /ʧ/ (“Cheltic”)
Old French pronunciation: /ʧ/ (“Cheltic”)
Modern French pronunciation: /s/ (“Seltic”)
Old English pronunciation: /ʧ/ (“Cheltic”) or /k/ (“Keltic”) [not an OE word, so I don’t know which way it would have been borrowed]
Modern English: /s/ (“Seltic”), either as a borrowing from French or a spelling pronunciation, I don’t know. The traditional way to say it in Modern English, now used only in specific contexts like the sports team, because:
Modern English2: /k/ (“Keltic”) is the way academics and neopagans say it. Both groups are aware that the letter C cannot be rendered /s/ in any of the Celtic languages.* This pronunciation is now standard. Neither “Seltic” nor “Keltic” is wrong. “Keltic” is preferred. “Celtics,” however you say it, is stupid. Why not the Boston Celts? “Celtic” is an adjective and pluralizing it is idiotic.
*Except Gaulish inscriptions using that version of sigma in the Greek alphabet.
Anyone else picture the Celtics and the Rangers battling it out as rivals? Winning would really depend on home-court advantage, either the Celts trying to run around on ice or the Rangers hobbling along wood in their skates.
Side note for those of you who love being pedantic with your articles–it’s The World Bank. Not just World Bank, the ‘The’ is always used and always capitalized. (If your style manual relies on an organization’s internal usage guide. If you’re free to rewrite/recapitalize at your discretion, have at it.)
If I go to New York and talk about the Yankees, people will agree that I am using the correct form. If I go to Glasgow and talk about the Rangers, people will think I am a sassenach who has spent too much time in the US.
It’s just wrong, and your explanation is somewhat undercut by the fact you didn’t refer to “the Celtic.”