There is a long Christian tradition of using the Greek letters that spell Christ in various abbreviated forms. Christ in Greek is written with a Chi (X), a Rho (rather like a P) and an Iota (an I or i to you and me). Often the letters were combined to form both the letters, and a design; the graphic to the left of this paragraph is fairly typical of such graphic designs. Sometimes, the I was seen as the stem of the Rho; sometimes it was a separate letter. In the middle ages the name of Christ, or even the first three letters, was specifically seen as something for scribes and artists to illuminate with their very best work.

The page in the Book of Kells known as F.34R is often referred to as the Second beginning of Matthew; it's the verse from Matthew 1:18 that in English in the 1611 version begins "Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost." The Latin text, the one used most often in medieval manuscripts, beings "XPI autem generatio..." The page is a Chi Rho pages because it features the Greek letters Chi Rho and Iota. To those of us familiar with the modern English alphabet, the letters look like a large XPI. These three letters are the abbreviated Greek form of Christ's name. The Book of Kells is probably the single best known medieval manuscript; it was created by monastics in the ninth century, and presently resides in Trinity College Library, in Dublin.

Here's another example of a Chi-Rho page; this one is from the Book of Lindisfarne, produced in England sometime between 680 and 720. The Book of Lindisfarne is in the British library now. It's the beginning of the same verse in Matthew as the Kells Chi-Rho page. You can still see the Chi and the Rho, very clearly. You can click the image for a larger view.

Over time the Chi-Rho-Iota shortened version of Christ's name was itself abbreviated. Late in the Roman Empire, post Emperor Constantine's conversion to Christianity, Roman coins were made with the Chi-Rho-Iota graphic. Even the villas of wealthy Romans in England were decorated with Christin themed mosaics using Chi-Rho-Iota. Eventually, the three letters Xpi were themselves abbreviated. Using X or Xp as an abbreviated way to refer to Christ became so common that it was used to shorten Christmas to Xmas.