This is the Chi Rho symbol, a Christian symbol for the Greek word ΧΡΙΣΤΟΣ, Christos, which means Christ.
It is formed by overlapping the first two letters of the word ΧΡΙΣΤΟΣ, Chi (X) and Rho (P). It is one of the earliest forms of a Christian monogram, or a Christogram. Chi Rho, pronounced as KEE-roe.
In Christian art, the Chi Rho is often flanked by the Greek letters alpha and omega, symbolizing Jesus Christ as the Alpha and Omega—the Beginning and the End. The Chi Rho symbol is not mentioned in the Bible, even though these letters are used to form the name “Christ” in Greek.
The Chi Rho symbol was used by early Christians to symbolize both Jesus and Christianity. It is attributed to Roman Emperor Constantine I, who used it as a military symbol. The first public use of the Chi Rho symbol was on 28 October 213 A.D., on the shields of Constantine’s warriors in the Battle of Milvian Bridge. Constantine’s troops saw a miraculous sign in the sky the day before the battle and Emperor Constantine believed he won the battle because of the Chi Rho symbol on the shields of his men. Several months later, in the Edit of Milan, Constantine declared Christianity legal in the Roman Empire, ending nearly three hundred years of Christian persecution.
This symbol appeared on the coins of Constantine and his successors.
Even though Christians have used the Chi Rho symbol to represent Christ and Christianity for centuries, Chi Rho has also been used as a good-luck charm and as a talisman for warding off evil. It is believed the symbol was worshiped by pagans in ancient Greece, believing the symbol bestowed good fortune.
The Chi Rho symbol exists today in numerous variations. Some of our altar cloths have this symbol.
The symbol may have lead to the modern practice of using the letter X in Xmas, as an abbreviation for Christmas.