Put the X in Xmas

"Xmas is, though, a much more venerable abbreviation than many suppose. The X signifies the Greek letter chi, which was traditionally combined with P, or rho, to signify the name of Christ. Constantine instructed his soldiers to scrawl the letters on their shields before the Battle of the Milvian Bridge, his victory in which led to the unlikely establishment of a Christian empire. Far from a symbol of secularization, then, Xmas carries echoes of the clash of battle that inaugurated political Christendom.
"The abbreviation’s use in English dates back to 1021, when an Anglo-Saxon scribe saved himself space by writing XPmas. The P was dropped but the term soldiered on: Coleridge used Xmas in his letter writing. The wags at Punch pressed it into service as a verb, Xmassing (one imagines the verb would get more use if the WASPs who currently spend their time summering, wintering, and weekending were somewhat more observant)."

Matthew Schmitz at First Things explains Xmas.