Xmas is a common abbreviation of the word Christmas. It is sometimes pronounced, but Xmas, and variants such as Xtemass, originated as handwriting abbreviations for the typical pronunciation. The 'X'
A 1922 advertisement in Ladies' Home Journal for Biberman Bros.: "Give her a L'Aiglon for Xmas"
"Xmas" used on a Christmas postcard, 1910
The Chi Rho is one of the earliest forms of the Christogram, formed by superimposing the first two letters—chi and rho —of the Greek ΧΡΙΣΤΟΣ in such a way that the vertical stroke of the rho intersect
Xmas
…usage dates back to the 16th century, and corresponds to Roman Catholic, Eastern Orthodox, Lutheran and Anglican liturgical use of various forms of chi-rho monogram. In English, "X" was first used as a scribal abbreviation for "Christ" in 1100; "X'temmas" is attested in 1551, and "Xmas" in 1721. The term…
Missorium depicting Emperor Constantine the Great's son Constantius II accompanied by a guardsman with the Chi Rho depicted on his shield (at left, behind the horse)
Coin minted during reign of Ptolemy III Euergetes in Egypt (from 246 BC to 222 BC). Chi Rho visible between bird's legs.
This coin of Crispus, son of Constantine, with a chi rho on the shield (struck c. 326) shows that the symbol mentioned by Lactantius and Eusebius was a chi rho.
The so-called "Chrismon of Saint Ambrose" (Chrismon Sancti Ambrosii), on display on the eastern wall of Milan Cathedral, a Chi-Rho combined with Alpha and Omega in a circle. According to Landulf of Milan (12th century), it was used by Saint Ambrose to introduce the catechumens to the mysteries of the Christian faith (whence it was called "oracle" or chresmos of St. Ambrose, written by Landulf as crismon, whence the later Neo-Latin term for the Chi-Rho symbol).