It is not a sin to use "X" in the abbreviation of Christmas. "Xmas" is not a disrespectful modern secularization, but derives from Chi (X), the first letter of the word "Christ" in Greek, the language in which the New Testament was written.
The symbol has been used by the church itself at least as far back as the 15th Century, when typesetters employed it with the newly invented Gutenberg press.
When fused with the second letter of Christ's name in Greek, Rho (P), it is called the labarum, a sort of sacred monogram symbolizing Jesus Christ. This ligature is said to have originated with Constantine I, who reportedly dreamed the symbol before the pivotal Battle of the Milvian Bridge (312). He ordered his soldiers to paint it on their shields, and when they emerged victorious from the battle, Constantine converted, becoming the first Christian Roman emperor.
But all this has little to do with the true meaning of Xmas. For that, you need look no farther than this year's magnificent Postal Service Christmas stamp. It is taken from a painting by the Italian master Sandro Botticelli which hangs in the Cleveland Museum of Art. The Virgin and Child With the Young John the Baptist was painted around the same time the holy "X" was first being inserted by frugal German typesetters into Gutenberg's invention.
What does "Xmas" truly mean? Click to enlarge and feast your eyes--you'll be brought as close as you can come to an answer.