For the ancient Christians, the name Christ was Greek for the Hebrew word Messiah, which meant "anointed one." The early Christians believed that Jesus of Nazareth was the long-awaited-for Jewish Messiah come to save Israel and be a light to the nations. In Greek, Christ is spelled with an X (instead of our "eks" pronunciation in English, in Greek it has a hard "k" sound to it: Xristos).
The sign of Christ was an X to the ancient Christians.
For us, we get to reclaim the name Xmas. We know what the X stands for - it's the ancient sign of Christ. Maybe there are some shopowners and other overly sensitive types you use the phrase "Xmas" as a generic way to share holiday cheer. But we know what the word really means!
So keep X in Xmas!