X used as shorthand in 'Christian' has a long lineage. It's the Greek letter Chi, as seen in the Chi-Rho symbol scrawled in the catacombs in Rome in the second century, which stood for the first letters of the word Christos, or 'anointed one'. To be honest (as an atheist), I really don't think any true Christian would feel upset about being reminded of the two millennia of history behind his or her faith.