More about the meaning of the letters "XP"

The third century Church used the Greek Letters Chi ("X") and Rho ("P") – the first two letters in the Greek speaking of the word "Christ," and sometimes "X" by itself to represent Christ. In religious art of the Renaissance and Medieval periods it was quite common to see these intertwined letters "X" and "P" in paintings of Christ. It is believed that this is the origin of the use of the letter "X" in "Xmas."

Online encyclopedia articles explain more:

The word "Christ" and its compounds, including "Christmas," have been abbreviated in English for at least the past 1,000 years... "Christ" was often written as "XP" or "Xt"... This X and P arose as the uppercase forms of the Greek letters χ and ρ used in ancient abbreviations for Χριστος (Greek for "Christ"), and are still widely seen in many Eastern Orthodox icons depicting Jesus Christ. The labarum, an amalgamation of the two Greek letters, is a symbol often used to represent Christ in Catholic, Protestant, and Orthodox Christian Churches.

The Oxford English Dictionary (OED) and the OED Supplement have cited usages of "X" or "XP" for "Christ" as early as 1485. The terms "Xpian" and "Xtian" have also been used for "Christian." The dictionary further cites usage of "Xtianity" for "Christianity" from 1634.