Let's rewind about 2000 years when parts of the New Testament were being written and recorded. From 50 AD to 100 AD, the scholarly language of the time was Greek and so most of this portion of The Bible was written in Greek. The earliest Greek symbols for Christ was cristoV where the first two letters were chi (c or C) and rho (r or R).
So the symbol for Christ in the Greek became XP and was later shortened to just X.
In the fifteenth century, Johannes Gutenberg invented the printing press. Because the words were set by hand, the cost of printing was extremely expensive. The church, in an effort to reduce production costs, used the abbreviations X for Christ and Xmas for Christmas.
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