From the Christmas Forum:
Xmas is not of modern coinage. The Oxford English Dictionary documents the use of this abbreviation back to 1551. Undoubtedly it was employed before that. Now 1551 is fifty years before the first English colonists came to America and sixty years earlier than the completion of the King James Version of the Bible! Moreover, at the same time, Xian and Xianity were in frequent use as abbreviations of Christian and Christianity.
You see, the X in Xmas did not originate as our English alphabet's X but as the symbol X in the Greek alphabet, called Chi, with a hard ch. The Greek Chi or X is the first letter in the Greek word Christos. ...
Gration claims that as early as the first century the X was used as Christ's initial. Certainly through church history we can trace this usage. In many manuscripts of the New Testament, X abbreviates Christos (Xristos). In ancient Christian art X and XR (Chi Ro--the first two letters in Greek of Christos abbreviate his name. We find that this practice entered the Old English language as early as AD 100. Moreover, Wycliff and other devout believers used X as an abbreviation for Christ. Were they trying to take Christ away and substitute an unknown quantity? The idea is preposterous.
Some may use Xmas today as an unchristian shortcut for Christmas, but the ancient abbreviation by no means originated as such. The scribes who copied New Testament manuscripts had no intention of taking Christ out of the New Testament. They used the abbreviation simply to save time and space. ..
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