The second-most-famous vision of a cross in the sky

Cross in the sky over Jerusalem - as imagined by Jan Luyken, ca. 1700.

Most people are aware of the famous vision seen by Constantine the Great of a cross in the sky with the words, "In hoc signo, vinces" — by this sign you will conquer. Having seen this miraculous vision, Constantine would cause his soldiers to march under Christian symbols, be victorious in his wars, and eventually convert to Christianity. Many citizens of the Roman Empire would follow him.

But not all.

During the reign of Constantine's sons, paganism continued to be practiced throughout the empire. Indeed, thanks to the weak rule and the rumored personal scandals of Constans, a pagan usurper named Magnentius was able to rise in the West in AD 350. Magnentius's henchmen soon captured Constans and put him to death. This dangerous uprising would have to be confronted by the last remaining son of Constantine, Constantius II, the Augustus of the East.

While these political and military maneuverings were going on in the West, a miraculous event occurred in the East in AD 351. In Jerusalem, another cross appeared in the sky, this time huge, long lasting and unmistakable. Here is how the event is described by the historian Hermias Sozomen writing in the 440s AD:

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At the time that Cyril administered the church of Jerusalem after Maximus, the sign of the cross appeared in the heavens. It shone brilliantly, not with divergent rays like a comet, but with the concentration of a great deal of light, apparently dense and yet transparent. Its length was about fifteen stadia from Calvary to the Mount of Olives, and its breadth was in proportion to its length. So extraordinary a phenomenon excited universal terror. Men, women, and children left their houses, the market-place, or their respective employments, and ran to the church, where they sang hymns to Christ together, and voluntarily confessed their belief in God. The intelligence disturbed in no little measure our entire dominions, and this happened rapidly; for, as the custom was, there were travelers from every part of the world, so to speak, who were dwelling at Jerusalem for prayer, or to visit its places of interest, these were spectators of the sign, and divulged the facts to their friends at home. The emperor was made acquainted with the occurrence, partly by numerous reports concerning it which were then current, and partly by a letter from Cyril the bishop. It was said that this prodigy was a fulfillment of an ancient prophecy contained in the Holy Scriptures. It was the means of the conversion of many pagans and Jews to Christianity. [Taken from The Ecclesiastical History of Sozomen, Book IV, Chapter VI]

Sozomen likely picked up his description of this event from an eye-witness account written by Saint Cyril, Bishop of Jerusalem. Cyril wrote a letter to Constantius announcing the advent of this strange apparition which he himself had witnessed. His letter has come down to us, and contains some additional details:

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The trophy of victory which our Lord and Savior Jesus Christ, the only-begotten Son of God, won over death—I refer to the blessed Cross—has been seen flashing like lightning over Jerusalem. 

In these holy days of the Easter season, on 7 May at about the third hour, a huge cross made of light appeared in the sky above holy Golgotha extending as far as the holy Mount of Olives. It was not revealed to one or two people alone, but appeared unmistakably to everyone in the city. It was not as one might conclude that one had suffered a momentary optical illusion; it was visible to the human eye above the earth for several hours. The flashes it emitted outshone the rays of the sun, which would have outshone and obscured it themselves if it had not presented the watchers with more powerful illumination than the sun. It prompted the whole populace at once to run together into the holy church, overcome with fear and joy at the divine vision. Young and old, men and women of every age, even young girls confined to their rooms at home, natives and foreigners, Christians and pagans visiting from abroad, all together as if with a single voice raised a hymn of praise to God’s Only-begotten Son the wonder-worker. They had the evidence of their own senses that the holy faith of Christians is not based on the persuasive arguments of philosophy, but on the revelation of the Spirit and power; it is not proclaimed by mere human beings but testified from heaven by God himself. [Excerpted from: Yarnold: Cyril of Jerusalem]

What is perhaps most interesting about the above accounts is that we see here yet another occasion where, apparently, a mass conversion of pagans and Jews to Christianity took place as a direct result of witnessing a miraculous event.

As for Constantius, Cyril assures him in the abovementioned letter that the miracle will allow him to "face his enemies with greater heart." Later that year, in September AD 351, Constantius II and Magnentius would clash in the epic battle of Mursa Major in Dalmatia. Magnentius was defeated and forced to retreat. He would survive another two years before meeting his end after the battle of Mons Seleucus in Gaul.