What does the symbol of the P with an X through the bottom of it mean?
You are speaking of the chi-rho monogram. Though the two letters look like P and X in the English alphabet, they are actually chi (looks like X) and rho (looks like P) from the Greek alphabet. They also happen to be the first two letters of “Christ” in Greek (Christos). Hence the chi-rho monogram is used as a symbol of Christ, Christianity, and Christians.