R. C. Sproul explains the origin of using "X-mas" when writing. As to why American Christians tend to protest the use ("You're taking the Christ out of Christmas!"), in actuality, the "X" is a clear reference to "Christ" in the Greek language. Says Sproul:
First of all, you have to understand that it is not the letter X that is put into Christmas. We see the English letter X there, but actually what it involves is the first letter of the Greek name for Christ. Christos is the New Testament Greek for Christ. The first letter of the Greek word Christos is transliterated into our alphabet as an X. That X has come through church history to be a shorthand symbol for the name of Christ.
We don't see people protesting the use of the Greek letter theta, which is an O with a line across the middle. We use that as a shorthand abbreviation for God because it is the first letter of the word Theos, the Greek word for God.
The idea of X as an abbreviation for the name of Christ came into use in our culture with no intent to show any disrespect for Jesus. The church has used the symbol of the fish historically because it is an acronym. Fish in Greek (ichthus) involved the use of the first letters for the Greek phrase "Jesus Christ, Son of God, Savior." So the early Christians would take the first letter of those words and put those letters together to spell the Greek word for fish. That's how the symbol of the fish became the universal symbol of Christendom. There's a long and sacred history of the use of X to symbolize the name of Christ, and from its origin, it has meant no disrespect.
So I guess if we protest "X" we have to protest all those fish stuck on Christian's cars ;-).
