The word “Christmas“ is a contraction meaning “Christ’s mass”. It is derived from the Middle English “Christemasse”.
The word “Mass” means death and was coined originally by the Roman Catholic Church, and belongs exclusively to the church of Rome.
“Xmas” is a known abbreviation of the word “Christmas”. Since the mid-sixteenth century the Roman letter X, was used as an abbreviation for Christ.
According to a book by Jeff Rovin (”Did you ever Wonder...”) the word for Christ in Greek is Xristos. The use of the shortened form “Xmas” was very popular in Europe in the 1500s. In our days “Xmas” is so common in advertising most likely because the word “Xmas” is significantly shorter than Christmas.