The monogram that looks like a combination of a P and an X is actually the first two letters of the Greek word for Christ – Chi (X) and Rho (P). The symbol was used by early Christians and is attributed to the Roman Emporer Contstantine, who used it as a military symbol. You will often see it on banners, called labarum, in ecclesiastical processions. The Chi Rho symbol is also the origin of using the abbreviation Xmas for Christmas.
Photo: from the Lilly family gravestone, Mount Olivet Cemetery, Wheat Ridge, Colorado