As a teen, I thought "Xmas" was a cute way to shorten "Christmas" (and it definitely took less time to write out!).
But then someone pointed out a horrifying concept: by using such shorthand, I was crossing Christ out of Christmas! Henceforth, I felt guilty anytime I used it. Anytime I saw "Xmas," I cringed.
There were times that writing out the whole thing was tedious, however, such as on a big plastic storage tub for the basement with tree ornaments. But I couldn't get myself to use it. So I'd compromise and use "Cmas." At least that started with Christ's first initial . . .
Turns out that all my worries were for naught. The term "Xmas" is very religious and very much keeps Christ in Christmas.
It goes back to the fact that the Greek word for Christ begins with the Greek letter Chi, which looks like, yep, the Roman letter X.
The Early English adopted that X as a symbol for Christ, so you'd see them everywhere denoting the Savior of the world: in symbols from religious art to carvings in church buildings and on documents and more. Sometimes the X all by itself was used as the name for Christ in writing.
Many early Christians (commonly known as Xians) considered the X to be further significant because not only was it the first letter of Christ's name in Greek, but it looked like the cross.
So literally, Xmas meant Christ's mass. It wasn't a shorthand for anything. When Christians centuries ago saw that word, they thought of Christ. They literally saw a symbol of his death in the cross. They didn't see it as keeping Him out of Christmas at all, but putting Him right in the middle of it.
I hope all of you have Merry Christmases and that Christ is the center of them, however you decide to spell the holiday.