For various reasons, there was no clear successor to the Roman Empire after the death of Emperor Constantius in 306 CE. In the following years, two clear candidates emerged: Maxentius, who held Rome and made himself emperor, and his brother in law Constantius’ son Constantine who was in Britain at the time. In 312, Constantine gathered his legions and marched to the Italian peninsula to challenge the usurper.
On the night of 27 October, Constantine said he had a vision (some accounts say his legions saw it also) of a symbol, and heard the words “Under this sign you shall conquer”. Although commonly thought to be the cross, the symbol was the early Christian Chi Rho (P with an X in the stem) made of the first two letters of the word “Christ”. That night Constantine had the symbol painted on his legion’s shields, helmets, and banners.
The next morning, Constantine’s legionnaires met Maxentius at Milivian Bridge over the River Tiber on the Via Flaminia. Constantine decisively defeated Maxentius, and killed most of his troops, including the usurper himself, as they tried to flee across the bridge or swim the river. Constantine claimed divine intervention of the Christian God as the reason for his victory.
Constantine was not a Christian himself. Like most Roman soldier-emperors he worshipped Sol Invictus and Mithras, but saw the Christian god as one of many, and for the rest of his reign he ended the persecution of Christians. Emperor Constantine I did much to promote and protect Christianity across the Empire and was baptized a Christian on his deathbed. Constantine is arguably the single most important secular reason for Christianity rising from a mostly Eastern slave, outcast, and women’s cult, to the state religion of both the Western and Eastern Roman Empires.