Diocletian initiates a great persecution of the Christians, and his reformed Roman empire undergoes the ultimate test as the time for succession arrives.

This is the Chi Rho symbol. It’s formed by putting the first two letters of Christ’s name in Greek – Χριστός – over one another.

Image from Wikimedia Commons user Dylan Lake.

Here you can see a coin with Constantine’s face on one side and, on the other, the Chi Rho symbol set atop his battle standard spearing a serpent.

Image from Wikimedia Commons, shared with permission from the Classical Numismatic Group.