Mike Bird has an interesting little post with which I cannot agree. He suggests, in short, that it’s ok to abbreviate Christmas with Xmas, suggesting -I think erroneously- that X is the Nomina Sacra. But it isn’t.
X (the English letter) is not equal to Χ (the Greek letter). Ξ is. Further, really, how many people know the Greek letter Χ and use it, and know that they’re using it in reference to Christ? Finally, the abbreviation for χριστος in the nomina sacra is χρ with the line above, never, to my recollection, is it simply χ (though I’m happy to be shown the error of my ways if someone can provide an example).
In sum, Xmas has nothing to do with Nomina Sacra, nor, in fact, does it have anything to do with Christ, or Christmas. It’s merely laziness. Come on people, ditch the annoying and meaningless abbreviations. Make a little effort and spell out Christ. It won’t kill you.