well we have 2 versions now...mine is from a film or "church story book" from long ago.... I can visualize the pictures of "troops" storming through the city...I hesitated to say German Youth Gang and Naxi symbols since I could be way.... centuries ...wrong. I am sure someone else can fill in the details
No, there is only one version and it's as simple as what I've said. "X" (as in chi) was used as an abbreviation for Christ from early times, perhaps initially as a camouflage for the religion. It was the first letter of the word Christos (meaning "the anointed one," e.g., the Messiah) and fortuitously was cross-shaped, so there seemed to be some symbolism or double meaning. It's been used as a scholarly and not-so-scholarly abbreviation since.
http://www.straightdope.com/mailbag/mxmas.html