...the abbreviation predates by centuries its use in gaudy advertisements. It was first used in the mid 1500s.
X is the Greek letter “chi,” the initial letter in the word Χριστός. And here’s the kicker: Χριστός means “Christ.” X has been an acceptable representation of the word “Christ” for hundreds of years. This device is known as a Christogram.
I will keep using the "X" spelling though. It pisses off the religious right. Hee hee.