XmasChristmas. It is derived from the word ΧΡΙΣΤΟΣ, transliterated as Christos, which is Greek for Christ. Greek is the language in which the whole New Testament was written.
Originally, Xmas was an abbreviation where the X represents the Greek letter chi, which is the first letter of Christ's name. However, because of the modern interpretations of the letter X, many people are unaware of this and assume that this abbreviation is meant to drop Christ from Christmas.
The abbreviation is widely but not universally accepted; some view it as demeaning to Christ.