It is argued that when Constantine I adopted what is now called a chi-rho device as the symbol of his rule in Gaul during the period 310-12, he intended it not as a Christian device combining the Greek letters chi and rho in abbreviation of the name of Christ, but as a radiate monogram combining the Latin letters P and R in celebration of the god Sol as the protector of the Populus Romanus ‘The Roman people’.