What the two above posters have written is exactly correct. In the early Christian period "X" was used as a symbol for Christ, and it still is in many church setting. X is by no means in the meaning of "X" or "ex" we have now. It's a Chi.
Have you ever been to a church and noticed XP embroidered anywhere? That's why. We have XP ornaments all over our church Chrismon tree, too.