The letter X in "Xmas" is actually the Greek letter "Chi", which is the Greek abbreviation of the word "Christ".

The letter X in "Xmas" is actually the Greek letter "Chi", which is the Greek abbreviation of the word "Christ".
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In Plato's Timaeus, it is explained that the two bands that form the soul of the world cross each other like the letter Χ.Chi or X is often used to abbreviate the name Christ, as in the holiday Christmas (Xmas). When fused within a single typespace with the Greek letter Rho, it is called the labarum and used to represent the person of Jesus Christ.

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