In the Greek language, which is the original language of the New Testament, the word Christ begins with the Greek X. X in Greek is Ch in English -- hence the first letter of the word Christ. (The entire word is XPICTOC.)
X is the Greek letter for ch, more guttural than the English k. It is the same sound as in the Scottish loch and the German achtung. In Greek, Christ is spelled XPICTOC. (X = ch, P = r, and c = s.) It sounds like hreeSTOS. The X in Xmas is simply shorthand for Christ.
So when folks say that the X is antichristian, they have misunderstood its etymology. Which is why I will continue to write "Xmas," without intending or even offering any disrespect to my Lord.
There is nothing irreverent about using this popular shorthand for Christmas. Tell your friends it's okay!