It’s rare for an emperor to earn the title ‘the great’. It’s even rarer for that same emperor to also be called a saint. So with Constantine I we’re dealing with no ordinary man. Now, while Constantine’s right to be called a saint is disputable – saints not being generally known for executing both son and wife – what is not open to question is that he was the greatest emperor of the late Roman Empire.

What’s more extraordinary is that Constantine came from, on his mother’s side, such humble stock. The Empress Helena was probably a tavern maid when she met an ambitious soldier called Constantius. Whether they ever married is not known for certain but we do know that this tavern maid left her home town of Drepanum in Asia Minor and accompanied Constantius on his campaigns, bearing him a son c.272AD. Constantius’s career took him virtually to the top of the military and imperial tree, culminating with the command of the provinces of Gaul, Britain and Spain. And there alongside him was his son, Constantine. But on 25 July 306 Constantius died at Eburacum (York), while campaigning against the Picts, and the legions acclaimed his son their Augustus.

At this time the Empire was ruled by four emperors, two in the west and two in the east, so Constantine was by no means the only man aspiring to the purple. However, there were more than four pretenders to the thrones, but illness and intrigue soon whittled the field down to four, with Constantine and Maxentius left as Augusti of the west. Not, you’ll guess, the most stable of arrangements, since kings generally dislike sharing their crowns.

Sure enough, in 312 the dispute between the emperors of the west turned bloody, and Constantine crossed the Alps into Italy with a vastly outnumbered force of 40,000 men. Maxentius had Rome as his base but, rather than wait out Constantine, he came forth with some 100,000 men, prepared to do battle at the Milvian Bridge over the River Tiber, north of Rome.

On the night before the battle, according to one source, Constantine had a dream commanding him to place the sign of Christ on his soldiers’ shields. Another historian says that there was a vision of a cross of light in the sky, with the words ‘by this, conquer’. Whichever account is true, we know that on 28 October 312 the armies of Constantine and Maxentius met in battle, and that Maxentius’s forces were utterly defeated and the former Augustus drowned while trying to escape back south across the river.

Constantine was now the undisputed emperor of the western Roman Empire. Meanwhile Licinius, via the odd battle of his own, had become the sole ruler of the eastern Empire. One of the first acts of the two emperors was to issue, in February 313, the Edict of Milan. This instructed the governors of all Roman provinces to stop the persecution of Christians (which had been vigorous under the previous emperors), to restore confiscated property to Christians, and to allow all citizens the freedom to practise whatever religion they wished

But history shows that it’s generally difficult to solve the equation of two emperors into one empire. Licinius reneged on the Edict and began a new persecution of the Empire’s Christian population. Three battles later, in 324, Constantine was the sole ruler of the Roman Empire, both east and west.

During the course of the war with Licinius Constantine had to lay siege to a city strategically sited on a promontory of the Bosporus. Whoever ruled the city was master over the sea traffic in and out of the Black Sea, as well as commanding a key position on the trade routes both north-south and east-west. If any one place could claim to be the centre of the world this was it. And on 8 September 324, only weeks after his victory over Licinius, Constantine laid out the boundaries of his new city. On this site New Rome would rule an empire for a thousand years after the fall of Old Rome.

But what about being a saint? There’s that unfortunate business with Crispus, his illegitimate first son, and Fausta, Constantine’s wife and mother of his later children to deal with. In 326 Constantine executed Crispus and then, not long after, the emperor had his wife, Fausta, killed too (reputedly by having her put in an overheated bath). Why did he kill two people so close to him? It is impossible to know for sure now, but perhaps the most plausible suggestion is that Fausta, wanting to ensure that her own sons succeeded to the throne, accused Crispus of attempting to seduce or rape her. Constantine, who was not known for calm forbearance, flew into a rage and executed his son. Then, when he found out that his wife had lied to him, he killed her too.

So, perhaps not a saint. But Constantine the Great? Yes, certainly.