this is actually a misconception, "X" comes from the Greek letter Chi, which is the first letter of the Greek word.... (looks like the forum wont accept greek, but it looks like.." XPiotoc"... those are just the english letters that look like greek letters), translated as "Christ". X- Christ the rest of the greek word-- Mass as in the church service.
X also represents the CHI-RHO which is another greek symbol for Christ. As we all know the New Testament was written in Greek.. So knowing this you can now use Xmas with no guilt.
Merry Xmas
sorry... 
