As a fan of the show Futurama, I often pay homage to it by calling Christmas "Xmas." Christians, who don't watch Futurama, miss the reference and think I'm insulting them and Christ by taking Christ out of Christmas. Once upon a time I thought that too. I thought that the X was crossing out Christ from Christmas.
Then I started studying Greek.
It turns out that the Greek word for Christ starts with the character "chi" (pronounced with a sound that doesn't exist in English). The letter "chi" is written like the English "X". So, writing "Christmas" as "Xmas" is more of a short form. According to Today's Article, this has been going on for centuries. In fact,t he letter "chi" came to represent the word "Christ". The word "Christian" could be written "Xian".
Just like I suspected. This article just confirms it.