The “X” in the short form of Christmas is not the letter x. It is the first letter of “Christ” in Greek, the letter “chi,” which looks like an x but is pronounced /kai/( rhymes with “sky”).
The second letter in the Greek word for “Christ” looks like a capital p but is pronounced “roh.” The first two letters together form the “Chi-roh,” which looks like a capital x with a capital p on top:
The Chi-roh has been used for over 1,500 years as a symbol of Christ. It can be seen here in the central part of the Hinton Saint Mary Mosaic, a representation of Jesus Christ that was found in England and has been dated to the early 4th century:
Hinton Saint Mary Mosaic – central roundel (early 4th century), British Museum, London – Photo by J Miall
Hinton Saint Mary Mosaic (early 4th century), British Museum, London – Photo from britishmuseum.org
Simply put, “XP / Chi-roh” or “X / Chi” are the initials of the word “Christ” in Greek, and that’s why Christmas becomes Xmas.