The idea of using ‘X’
in place of Christ is not a modern idea. In the Modern Roman Alphabet, the first letter of the word ‘Christ’ is ‘chi’ which is
represented by a symbol similar to the letter ‘X’.
You will frequently see
people write Xmas as well as Xian, which means Christian, using the same
principle. [Cross and Chi -- Two chi's superimposed
upon one another. The chi in its normal position represents Christ. The chi in the position of the cross represents Christ's crucifixion.]
Source: Paul G. Donelson
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