We often associate the term "X-Mas" with an American origin. But is the shortened spelling actually based on American roots?
Christmas card "I bring you a Merry X-Mas" ca. 1910
The answer at this point is: No! The X in this spelling does not come from the Latin alphabet, but is the Greek letter "Chi" (Χ) to assign. The word ΧΡΙΣΤΟΣ (Christos) begins with just this and Χ is only the abbreviation and symbol for Jesus Christ. Together with the second letter "Rho" (Ρ) it forms the Christ monogram ☧.
The letter "Chi" was often used in old English abbreviations for the syllable "Christ". For example, "Xian" was a common abbreviation for Christian, "Xianity" stood for christianity. The earliest word relationship to X-Mas is probably the word "X'temmas" (pronounced christemmas) and goes back to the year 1551.