I believe the X represents a cross. Supposedly when Europeans were immigrating to Ellis Island Christians were labeled with an X cross and Jews were labled with a O circle. I might not have this next part exactly right but in Yiddish a circle or O is a “Kike”. Which is where that word originated. They gave Christians a X for obvious reasons and Jews were given an O (probably because its pretty opposite looking).
Depending on when XMas was first used, it is my belief that it simply represents the X (cross) which was shorthand for Christian.
I’m afraid you are mistaken. As noted in the article, X - the Greek Chi - has been used since the beginning of Christianity. An X-shaped cross would be a St Andrew’s Cross.
Why would Christians be labeled at all? As the overwhelming majority of immigrants, wouldn’t it make much more sense to simply not label them at all? And that’s even assuming that there were any labeling of immigrants by religion at all.
Knowing what’s true and identifying stuff that’s not are two different things.
Leo Rosten’s story is doubtful at best. It comes from The Joy of Yiddish. The story sounds exactly like folk etymology. It seems plausible, except that it depends on something happening on Ellis Island that is hard to confirm from any source that doesn’t obviously derive from Rosten. Jewish immigrants were not especially illiterate in the first place. Italian immigrants had a much higher level, perhaps twice as high. (That book doesn’t mention kike, a startling omission if the Xs and Os story has any validity.)
And then there’s the provenance for the tidbit. It’s given as fourth-hand evidence, though the last leg is through Stephen Birmingham. Which is odd, because earlier on that page - and on your very own Wikipedia cite - we are told that Stephen Birmingham gives a different etymology for kike. Did he ever recant? I’ll give you the answer: no. He repeated the story in 1984’s The Rest of Us: The Rise of America’s Eastern European Jews and didn’t mention Rosten’s pretty little fable at all. That’s a decade and a half later.
Various other, and also much earlier, possibilities exist. Cassell’s Dictionary of Slang (which gets quoted uncredited online a lot) e.g. throws cold water on it.
Rosten was a fine writer and an interesting one. A historian he was not.
There’s no possibility that X in Xtian represents the cross, of course. I’m sure you don’t doubt that.