Constantine's vision, the cross of light before the Milvian Bridge

312 CE. Constantine, Western Augustus, was on campaign against his rival Maxentius. The decisive clash would come at the Milvian Bridge, on the Tiber just north of Rome, on 28 October 312.

According to Eusebius of Caesarea, the emperor had reflected that emperors who trusted many gods came to bad ends. He resolved to worship his father's single God, and prayed earnestly that this God would reveal who he was.

It was about midday. The sun was beginning to decline. Then, in the words of Eusebius, "he saw with his own eyes the trophy of a cross of light in the heavens, above the sun". It was "bearing the inscription, Conquer by this." In Greek, ἐν τούτῳ νίκα. The Latin in hoc signo vinces is a later rendering.

Constantine was struck with amazement. He was not alone. Eusebius says the whole army following him on the expedition saw it too. If that was the force that fought at the Milvian Bridge, it numbered an estimated 20,000 to 25,000 soldiers. Not one of them is named.

That night, Christ appeared to him in his sleep with the same sign and told him to make a likeness of it as a safeguard in battle. At dawn Constantine summoned goldsmiths and jewellers. The result was the labarum. A gold-covered spear with a crossbar, topped by a jewelled wreath holding the Chi-Rho monogram.

The story did not come straight from the field. Eusebius wrote it between 337 and 339, about 25 years later, and said he had it from Constantine himself, long afterwards, under oath. His earlier Ecclesiastical History called the victory God-given but mentioned no vision.

Lactantius, writing before about 315, puts nothing in the sky. "Constantine was directed in a dream to cause the heavenly sign to be delineated on the shields of his soldiers," he wrote. A panegyric of 310 tells instead of Apollo, accompanied by Victory, offering laurel wreaths at a temple in Gaul. In 321 the orator Nazarius described a heavenly army bearing flaming arms, led by the deified Constantius I, descending from the sky. The Arch of Constantine, dedicated about 315, credits the victory to instinctu divinitatis, "by the prompting of the divinity".

The leading natural explanation is a solar halo, whose rings, mock suns and sun pillars can draw a cross around the sun. In 1890 E. C. Richardson already suggested a "parahelion". Peter Weiss argued that the vision of Apollo and the cross were one halo display seen in Gaul in 310, read first as laurel wreaths and later as a Christian sign. T. D. Barnes called the consequences "momentous", while Raymond Van Dam called the idea "reductionist". Richard Flower noted that no one has recorded words appearing in a halo.

Others see literary invention, a dream, or political calculation. John Julius Norwich argued it never happened as told. In 2003 researchers proposed a meteorite impact at the Sirente crater in Abruzzo, about 100 km from where Constantine's army was said to be camped before the battle. Critics have since argued the crater is man-made, or a mud volcano.

The motto outlived its moment. It appears on coins of Vetranio and Constans II, in Portuguese royal heraldry, at the Vatican's Scala Regia, and with Sigma Chi since 1861. Painters from the Paris Gregory to Piero della Francesca and the School of Raphael returned to it. A cross of light. Above the sun. At midday. Seventeen centuries on, it is still argued over.