For those who have are concerned with people who use X in place of Christ when writing Christmas, you might find this article, by R.C. Sproul interesting. Let me add that when I was in seminary I began using X for Christ, JC for Jesus Christ, HS for Holy Spirit, G for God. In a strange way it was comforting and expedient. This allowed me to write quickly, but also when I would throw a piece of paper in the garbage I was never throwing out the complete name of God.
Yes, it may sound strange but some of that comes with my Jewish background and the sanctity of God's name. You see, Jewish people (practicing Jews and others as well) when writing the name God, will write G-D, this way they will never throw the name of God out. It's an interesting premise which stuck with me through seminary and remains with me to this day. I don't even like deleting God's name from a sentence I write on my computer.
Here is the article from R.C. Sproul
The simple answer to your question is that the X in Christmas is used like the R in R.C. My given name at birth was Robert Charles, although before I was even taken home from the hospital my parents called me by my initials, R.C., and nobody seems to be too scandalized by that.
X can mean so many things. For example, when we want to denote an unknown quantity, we use the symbol X. It can refer to an obscene level of films, something that is X-rated. People seem to express chagrin about seeing Christ's name dropped and replaced by this symbol for an unknown quantity X. Every year you see the signs and the bumper stickers saying, "Put Christ back into Christmas" as a response to this substitution of the letter X for the name of Christ.
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. 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.
Taken from Now, That's a Good Question!
©1996 by R.C. Sproul. Used by permission of Tyndale on the blog of Ligonier Ministries.