Contrary to popular belief, there's nothing wrong with saying "Xmas" instead of "Christmas." Some people would like to think that the word "Xmas" is a part of a grand conspiracy to take Christ out of Christmas.
While it's true that there are some people might use "Xmas" for that reason, the reason for the X being there has nothing to do with any "war on Christmas" or secret agenda to obscure the name of Jesus Christ.
Christ in Greek
In the original language of the New Testament, the word for "Christ" begins with the Greek letter chi, which basically looks like the English letter X. "Christ" in Greek looks like this: Χριστός.
X is Short for Christ
Dating back to the 16th century, people started using "Xmas" as a shorthand for "Christmas" because the X stands for "Christ." In fact, if you start looking into historical symbols used within the Christian church over the centuries, you'll find that using X as a symbol for the name of Christ is pretty common. Simple as that!
So the next time someone tries to snidely say "Xmas" to you, thinking they're seriously offending your faith, you can smile, reply with, "Merry Xmas!" because they actually just said the name of Christ anyway. LOL