The first chapter of Diane Moczar's book, Ten Dates Every Catholic Should Know, concerns the Edict of Milan, which legalized Christianity throughout the Roman Empire in A.D. 313.
The reign of Constantine followed hard upon the heels of the worst persecution Roman Christians had suffered, under the rule of Diocletian. Diocletian's persecution had begun in A.D. 303, and for ten years had been vigorously prosecuted throughout the entire empire.
Moczar describes this persecution: "All Roman men, women and children were summoned to sacrifice to the gods of Rome or die. Unlike some earlier persecutions that were localized in one or another of the provinces of the empire, this one was enforced everywhere." p. 3
The situation of the Christian in the Roman empire from the time of Nero in the first century until the reign of Constantine in the fourth century was one fraught with peril. In many ways, we seem to be descending into like times. See if you can recognize any emerging modern similarities with the following situation:
"..times of peace could change change into nightmares from one day to the next. Sometimes all it took was a few indignant idol-makers blaming Christians for a decline in business, or charges from actors furious that Christians were staying away from the obscenity and cruelty of the theaters. A riot could easily start, especially in cities crowded with idlers looking for diversion, and Christians would be the victims. A Christian could also be denounced by his neighbors: maybe he refused their dinner invitations so he would not have to give the customary libation to their household gods, or he kept his children home from the pagan schools. As St. Justin Martyr put it, 'The world suffers nothing from Christians but hates them because they reject its pleasures.'" p. 4
Into such a state of seeming hopelessness entered Constantine, just one pretender, and not the most likely one, to the the throne of the West upon the death of Diocletian. Constantine's main rival for the throne was his brother-in-law, Maxentius. The armies of these two leaders met at the Battle of the Milvian Bridge in 312, where Constantine's army was outnumbered more than 2 to 1.
Before the battle, Constantine experienced a supernatural vision, in which he saw (depending on the account) a cross, or a chi-rho symbol, in the sky and the Latin words for "in this sign, conquer." Eusebius reports that Constantine himself said Christ appeared to him and told him to make this sign his battle standard, as a pledge of victory. He did so, and his force of about 40,000 routed Maxentius' force of about 100,000. During the battle, Maxentius himself was drowned.
The next year, Constantine met with the Eastern Emperor, Licinius, in Milan and issued the famous Edict, finally legalizing Christianity, and ending the prospect of wholesale persecution within the Empire. No doubt he was influenced by his mother, St. Helena, because at the time of the Edict he was not yet Christian himself.
But, as Moczar highlights, the God of history intervened in an unexpected way to end a persecution of His Church, a persecution with the full weight of the decadent Roman Empire behind it. A persecution that looked final. "As it turned out, the Edict of Milan was to be the great charter of liberation for the Catholic Church: one of those stunning divine surprises that punctuate Catholic history." p. 10. This Edict allowed freedom for the Church and set the stage for one of the most important ecumenical councils in history, the Council of Nicea in 325. Also, St. Helena was able to journey to the Holy Land and rescue or preserve many of the most important relics and holy sites of Our Lord's ministry, passion and resurrection.
A further unforeseen event affecting Church history occurred when Constantine decided to move the capital of the Empire to the new city he founded, Constantinople, or New Rome, in modern day Turkey. The Popes were thus allowed to act with greater independence due in part to their physical distance from the Emperors.
Moczar's next date covers the decline of the Empire, the Hun invasion, and the response of Pope St. Leo the Great.
For more information, the complete text (in English) of the Edict of Milan can be found here: http://gbgm-umc.org/UMW/Bible/milan.stm
Dr. Moczar's book can be obtained here: http://www.amazon.com/Dates-Every-Catholic-Should-Know/dp/1933184159