Xian (abbreviation for Christian)
Xmas (abbreviation for Christmas, may be pronounced eks-mas)
Although neither of those is actually an X; it's really the Greek letter chi, corresponding to English "ch," and the initial letter of "Christ" in the Greek alphabet. (I've heard some religious types complain about "Xmas" as an abbreviation, believing that it was censoring Christ's name as part of the atheist "war on Christmas" or whatever, but actually it's just his initial in the very alphabet he himself would probably have written it in, given that Greek was the lingua franca
of the Eastern Roman Empire at the time he would've lived.)
the letter Chi, of course.
But, when either word is rendered in the Latin Alphabet, which is what we use, the letter is most definitely X, capital ex.
That is to say, the symbol was latinized