What is a chi-rho?

Answer 1

The Chi Rho is a symbol made up of the first two Greek letters that spell the name of Christ. It is an early Christian symbol allegedly seen by Constantine before his great battle at the Milivan Bridge. Constantine had the sign painted on the shields of his men and he went on to victory against the superior forces of his enemy. Convinced of Christ's power, Constantine thus put the sign upon the imperial standard. You can follow the link below to see the Chi Rho.

The only problem with this answer is that the chi-rho symbol was around before Christianity, on the coins of Ptolemy lll Euergetes 246-222 BC. I have added a link to the coin.