The Chi Rho, or Chi Rho Iota, is an ancient symbol for Christ and is found throughout church history in a variety of ways. The earliest use appears to be in the catacombs, along with several other symbols such as the ichthus (fish) and the anchor. The Chi Rho is formed by using the first two letters for Christ in the Greek language ΧΡΙΣΤΟΣ, which phonetically would sound like KRISTOS. Often an alpha and omega will appear, as in the picture above from Lullingstone villa (4th century).
The symbol is also called a Labarum by the Romans and is the symbol that legend says the Emporer Constantine saw in a dream in the year 312, which would lead him to victory. The Labarum is a military standard and can be found on roman shields and on poles that would hold banners or flags.
Both of the above used the Chi Rho as a monogram, affixing the letters in the same space to form a single symbol, and while this style of the symbol is still in use today it isn’t the only way in which the XP (Chi Rho symbol) was depicted. In the famous Lindisfarne Gospels and the Book of Kells, the xp is found as well.
The Book of Kells dates to the 9th century Ireland. It is also known as the Book of Columba, who was a monk in the 6th century that taught the Gospel in Ireland and Scotland. The Lindisfarne Gospels dates to the 8th century England and are attributed to a monk named Eadfrith who was bishop of Lindisfarne. Both are famous illuminated manuscripts and examples of how Sacred Scripture was preserved through the early middle ages.
The book of Kells as well as Lindisfarne are an Evangeliary, or Gospel Book. This is the book where the gospel is read from at mass. It’s cover has the four evangelical symbols for Matthew, Mark, Luke and John, and is usually made of gold, though both books lost their covers during Viking raids, taking the gold and leaving the real treasure. Both were written in Latin, which was the language of the people, and there is a translation of the Lindisfarne that has Old English which comes from the 10th century.
As can be seen on the ChiRho pages in both examples, the Book of Kells above and the Lindisfarne Gospels below, there is an entire page dedicated to the ChiRho, XP as well as the I (Iota), and are not found to be in the monogram style. They both are placed right at the beginning of Matthew 1:18 where in the genealogy listed, Christ enters and (the word is made flesh). In the Book of Kells the ChiRho page reads: XPI autem generatio.. which would translate as “Now the generation of Christ was in this wise…” and continues, “When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.”
If you get a chance, find a detailed pic of both of these and look at the amazing detail, the figures of people, animals, and the beautiful celtic style knots.