Not quite. X = "chi" in Greek, which is the first letter in the word for Christ, but it is only the first letter, not the whole word. While the "X", as in "Xtian" or "Xmas" is fairly common usage, this traditionalist agnostic still prefers to spell it out. I've always considered "Xmas" to be rude. Personal opinion only, but that IS my opinion.
Blessings on your Journey!