This is from the Holy Book of Wikipedia, but it is well known
Xmas is a common abbreviation of the word Christmas. It is sometimes pronounced /ˈɛksməs/, but Xmas, and variants such as Xtemass, originated as handwriting abbreviations for the typical pronunciation /ˈkrɪsməs/. The “X” comes from the Greek letter Chi, which is the first letter of the Greek word Χριστός, which in English is “Christ”.[1] The “-mas” part is from the Latin-derived Old English word for Mass.[2]
There is a common misconception that the word Xmas stems from a secular attempt to remove the religious tradition from Christmas[3] by taking the “Christ” out of “Christmas”, but its use dates back to the 16th century.