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.
Zwinglius Redivivus 
I think you’ll find that the OED released a statement on this a few years ago, and Xmas indeed derives from medieval χρmas, with the ρ dropped in later usage. So etymologically the ‘X’ in Xmas does indeed derive from Christos.
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and yet that’s not what mike said.
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I just read Michael Bird’s post, where he claims the abbreviation of Christos to “X” by “Early Christians”. Bird is incorrect on that particular point – the abbreviation made by early Christians is χρ.
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gee i just said that.
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Incidentally, I use editing software called LaTeX, where the “X” is a chi, and hence it is pronounced LaTech.
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From Larry Hurtado I understand that the usual two-letter contracted form of χριστος in the nomina sacra involves the first letter and the last letter. The latter varies according to the case. Sometimes there are three-letter contractions that include the rho. He has a couple of relevant essay (“The Meta-Data of Early Christian Manuscripts”) and chart available on his website, and his book The Earliest Christian Artifacts has a chapter on nomina sacra.
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