display | more...
Though we see it as the letter X (even sometimes say it as "X-mas"), it is actually from the Greek letter "chi" which has a sound similar to the one in words like "Christmas" or "Christ." "X" (chi) has a long history as an abbreviation for Christ from the early days of Christianity. It is the first letter in Christos (in Greek) which was from what "Christ" was derived (meaning "anointed one," "messiah").

From that comes the abbreviations often seen online: "xtians" and "xtianity" for "Christians" and "Christianity" (sometimes used with pejorative intent).