in early Christian time, a common symbol for the church was what looked like a pizza pie honestly and later just a weird looking x with a p on top. It contained Greek letters combine into one symbol with the x and what looked like a P on top. this spelled "Christos". the x-ish looking symbol would mark the spot for church structures so believers could worship there and the Romans had no way of knowing where to attack believers. After the invention of the printing press, several printers used the X to represent Christ because it cut down on expensive printing costs. Later, the ichthus (or Jesus fish) that is so common today replaced the Christian word-symbol. That's the story behind XMas...