Today, 28th October, is calculated to be the exact anniversary of the battle of the Milvian Bridge in Rome, where rival Roman Emperors (and brothers-in-law) Constantine and Maxentius personally led their armies against each other in the year 312. Maxentius made some bad tactical decisions and his forces lost badly to those of Constantine.
The victory allowed Constantine to become sole Emperor of the Western Empire, and during his reign Christianity was first legalized (in 313, along with all other non-Roman religions) and then gradually grew in respectability and importance. (It is commonly thought that Constantine made Christianity the Empire’s official religion, but that step occurred much later under Emperor Theodosius.)
Late in life, and struck with a serious illness, Constantine registered himself as a candidate for Christian baptism, a process which would have required a period of religious instruction. However, he died soon after. Later Christian sources embellished the story of his death to the point where it’s hard to work out what really happened, but it’s certainly possible that he was baptised a Christian before death.
Christians of the time, and in later centuries, were so grateful for Constantine’s support that his pro-Christian measures tended to be exaggerated and his adherence to traditional Roman religion ignored. As with many prior and subsequent saints, fictional episodes and embellishments were added to the story of his life.
One of these is probably the best-known story about Constantine. Prior to the battle at the Milvian Bridge, it was said, Constantine had a vision or dream, in which God told him to have his troops put a divine symbol on the front of their shields. In some versions, it’s the cross. In others, it’s the Chi-Rho monogram. Constantine was told “Through this sign, conquer.” And the rest is history.
The preceding paragraph isn’t history though. It’s a Christian fairy story. At the time of the battle, Constantine was promoting the Roman cult of Sol Invictus, “the invincible Sun”. After the victory, the new government of Constantine commissioned a triumphal arch opposite and framing the large statue of Sol Invictus, with sculptures relating the story of the battle.
Constantine’s victory arch still stands in Rome, and you can read the graphic novel strip of his exploits in the sculptures. And there, in the section showing the battle at the Milvian Bridge, you can see that the soldiers actually do have a divine symbol on the front of their shields. It’s the Sun sign of Sol Invictus.