During the reign of Emperor Nero in Rome a persecution had broken out
against the Christians living in that city. It caused them to flee to
the catacombs to meet in secret and to use code words for things of a
Christian nature. That symbol is what you may have seen with an "IXOYE"
inside of it on the back of someone's car. This was a secret symbol
Christian's used to identify each other then. The letters "I", "X",
"O", "Y", and "E" are actually an abbreviation in Greek letters
standing for Jesus Christ, Son of God, Savior.