That title doesn't say XP as in 'maximal projection', incidentally - it's the Greek letters chi and rho, which look infuriatingly similar to XP.
I was listening to the excellent A history of the world in 100 objects podcast from the BBC, and there was a programme about a very early representation of Christ, on a floor mosaic in Hinton St Mary.
Chi rho, I said to myself, wondering what the letter chi (pronounced kai) was, and then something occurred to me. I don't think I would have had this thought if I'd seen the words written, but because I was listening to audio, I heard chi rho and thought Cairo.
I wonder, I thought, if Cairo is for some reason named after old Jesus himself. Obviously Egypt is an Arabic country with Islam as its major religion, and so has less reverence for some minor prophet than Christian cities would have, but Cairo's had a long history of being pinched by various European nations on account of it being a nice city with a lot of wealth. Who's to say that we don't have our own name for it which is completely different from the Arabic name? It wouldn't be the first time (after all, we call Egypt Egypt, not Misr. See also Bangkok, Germany, Japan and Finland, among many others. Croatia, which I initially thought was another example, turns out to be from the same root as Hrvatska, which is what the Croatians call it).
So I checked Wikipedia and it's not. It's from the Arabic name for the city, al-Qahira (literally the Vanquisher). Ah well. Another theory quashed.