Why X is Used to Replace Christ in Christmas (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.