It isn't an "atheist" form at all:
Xmas has been used for hundreds of years in religious writing, where the X represents a Greek chi, the first letter of [the Greek word meaning] "Christ".
True, a youth movement here is called chiro, idd chi-ro. Wich is a christian movement (not sure what the ro means). One of it's symbols is P-X or Pax (peace) Christi ... peace of christ.
