Medieval scribes always abbreviated. When one is using a feather pen, dipped in ink laboriously made from oak gall and lamp black, writing on parchment that had to be carefully prepared from sheep skin, it's a slow process. One of the ways to pick up the pace was to abbreviate.
A number of abbreviations were standard from the ninth century on. For example, a short horizontal line drawn over a vowel meant that there was a following -m- or -n-. Hence, to write cu with a short horizontal over the -u- was to write the common Latin word cum (meaning with), without having to devote energy to all those little vertical lines (called minims) that go into the letter -m-.
Similarly, the letter -p- with a short horizontal through the lower leg (the descender) meant per, a word that came up a lot in medieval Latin. It could also be found in the middle of a word, so that imperator (emperor) would be written as impator with a short horizontal through the lower leg of the -p-.
One could even write episcopus (bishop) as epc, because, after all, what other word could it possibly be?
A very common abbreviation was of the word Christos. This is of course Greek, meaning "the anointed one" or "the Messiah," Christus in Latin, Christ in modern English. (The New Testament was written in Greek, though medieval writers used the so-called Vulgate, the Latin translation.) Because Christos is a Greek word, which medieval scribes recognized as such, they abbreviated it using Greek letters, even though very few of them knew much Greek at all.
The first letter of the word in Greek is "chi," what we would call a K-sound, written as X. Thus a capital X stood for Christ. Sometimes the scribes would add the second letter, the "rho," the R-sound, written as P. One will see on sarcophagi and altars the letters X and P superimposed on each other. This is the "chi-rho" (pronounced Cairo), meaning Christ.
So a scribe referring to Christians might write Xiani. Christmas was Xmissa (missa is Latin for mass).
This abbreviation system has continued in the modern Xmas. Some people find the abbreviation deeply disturbing, the reduction of Christ to a cipher, to an unknown. But this is not the case at all. (One does wonder if the mention of "the unknown" is a leftover from modern high school algebra.)
Medieval monks could never be mistaken for secularists, and they found the abbreviation very reverent. Indeed, there may have been some holdover of the ancient Hebrew idea that you should not actually say or write in too explicit a way (or at any rate write on something that might be discarded) the sacred name of God.
So if someone today loudly pronounces they do not believe in Christianity and will therefore refer only to Xmas, that's one thing. But you cannot read an anti-religion sentiment into the simple abbreviation. It does, after all, go back to the earliest existing western Bibles.
For more on the history of Christmas, see my essay "Contested Christmas," available as an ebook from Amazon and other ebook platforms.
© C. Dale Brittain 2015