“Xmas” and “X-mas” are common abbreviations of the word “Christmas”. They are sometimes pronounced “eksmas” and are not irreligious.

The “-mas” part comes from the Anglo-Saxon word for “festival” or “religious event”:
And the X probably arose from “XP” or “Xt”; which are found in Anglo-Saxon as far back as 1021 AD. This X and P arose as the uppercase forms of the Greek letters χ and ρ), used in ancient abbreviations for Χριστος (Greek for “Christ”).

Source – Wiki