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).