In medical terminology X can mean 'all the rest' such as in Hx means history on your medical chart.
There are lots of handy, geeky terms like that. Xmit=transmit. Wx=weather. Dx=Diagnosis.
But Xmas is His initial in the place of His name. I find it perfectly acceptable.
If "x" can mean "all the rest," is that why the main airport in Los Angeles is called LAX: Los Angeles and the rest of the cities in L.A. county?