You are speaking of the chi-rho monogram. X x-tilde median of a sample. X capital X a variable.
They also happen to be the first two letters of Christ in Greek Christos.
X is a symbol for multiplication. Texe Marrs says it is used a symbol for the Egyptian deity Osiris because you see it in Egyptian pyramids and temples while we find pharaohs buried with their arms and legs crossed in devotion to this sun god. 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. Probability that of events A and B.