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Conversion of Constantine according to Eusebius

Writing after Constantine had died, Eusebius claimed that the emperor himself had recounted to him that some time between the death of his father – the augustus Constantius – and his final battle against his rival Maxentius as augustus in the West, Constantine experienced a vision in which he and his soldiers beheld a Christian symbol, "a cross-shaped trophy formed from light", above the sun at midday. Attached to the symbol was the phrase "by this conquer" (ἐν τούτῳ νίκα, en toútōi níka), a phrase often rendered into Latin as " in hoc signo vinces ". In a dream that night "the Christ of God appeared to him with the sign which had appeared in the sky, and urged him to make himself a copy of the sign which had appeared in the sky, and to use this as a protection against the attacks of the enemy." Eusebius relates that this happened "on a campaign he was conducting somewhere". It is unclear from Eusebius's description whether the shields were marked with a Christian cross or with a chi-rho, a staurogram, or another similar symbol.

The Latin text De mortibus persecutorum contains an early account of the 28 October 312 Battle of the Milvian Bridge written by Lactantius probably in 313, the year following the battle. Lactantius does not mention a vision in the sky but describes a revelatory dream on the eve of battle. Eusebius's work of that time, his Church History, also makes no mention of the vision. The Arch of Constantine, constructed in AD 315, neither depicts a vision nor any Christian insignia in its depiction of the battle. In his posthumous biography of Constantine, Eusebius agrees with Lactantius that Constantine received instructions in a dream to apply a Christian symbol as a device to his soldiers' shields, but unlike Lactantius and subsequent Christian tradition, Eusebius does not date the events to October 312 and does not connect Constantine's vision and dream-vision with the Battle of the Milvian Bridge.