Beginning in medieval times, Christians writing in English sought to mimic the Greek approach by abbreviating “Christian” as “Xpian”—with the “Xp” visually evoking the old letter combination—or, eventually as “Xtian,” probably as the original association waned with time. And when the name of the holiday commemorating the birth of Jesus came to be called “Christmas,” a shortened version of “Christ’s mass,” it got further shortened with the Greek-style “X.”
