The Chi Rho symbol is not quite as rapidly identified with Christianity as the ichthys, but it is still a recognizable Christian symbol. Looking to modern eyes like a flattened ‘X’ appearing superimpose on a capital ‘P,’ the Chi Rho is formed by the combination of the first two Greek letters in the word “Christ,” chi and rho.

The Chi Rho symbol was first used by Constantine the Great in the fourth century. According to the historian Eusebius, Constantine saw the symbol in the sky before the Battle of the Milvian Bridge. The future emperor knew little of Christianity but swore he would follow the god that had spoken to him if he won the battle. Legend has it that Constantine had his men paint the symbol on their shields before they went off to battle. At the Milvian Bridge, Constantine was victorious and converted to Christianity shortly thereafter. This brought Christianity out of the shadows, and both the story and use of the Chi Rho sign spread like wildfire.