A curious occurrence happened in the early 4th century Roman Empire. The early church historian Bishop Eusebius tells the story of the Roman Emperor Constantine, who, before a battle against his rival Emperor Maxentius in 312 AD at the Milvian Bridge outside of Rome, had either a dream or vision that he was to conquer in the sign of Christ.
Explaining this to his troops, they made a battle standard of the Greek letters chi and rho, the first two letters of Christ in Greek, and were victorious. Constantine converted to Christianity and, in 313 AD, put into law the Edict of Milan, making Christianity a legitimate religion with toleration toward Christians across the Roman Empire. This ended the Empire-wide Great Persecution of the church under the earlier Emperor Diocletian.