On the 27th October 312 AD the battle of Milvian Bridge was fought near Rome. The two men contending for control of Rome and the Empire were Constantine, who controlled Gaul, England and parts of Germany and Maxentius, who controlled Italy, Spain and Africa.
It happens the two were brothers in law.
The day before the monumental and pivotal battle, Constantine saw a flaming cross in the sky, taking I as an omen, he had his men put a cross on their shields and banners.
Full of strength at what their commander had seen, Constantine’s men won the battle. Maxentius drowned crossing a bridge over the Tiber.
Rome would soon be unified under one ruler (only Licentius remained in Constantine’s way.)
Constantine would spend the ret of his reign Christianising his subjects.
He converted on his deathbed, becoming the first Christian Emperor, in 337 AD.