X is the Greek letter "chi" which is the first letter in "Christ. Consequently "Xty" or "Xtianity" is a scholarly abbreviation for Christianty, as "Xtian" is for "Christian", "Xt" for "Christ", and IHS (first three letters of "Jesus" in Greek) is for Jesus.
For those of us who cannot type their names w/o misspelling them such abbreviations are a mercy.