"He saw with his own eyes the trophy of a cross of light in the heavens, above the sun, and bearing the inscription, CONQUER BY THIS. At the sight he himself was struck with amazement and his whole army with him."
Quote taken from: Life of the Blessed Emperor Constantine by Eusebius Pamphilus, written ca. AD 337. The famous incident of the cross in the sky occurred in AD 312 prior to the Battle of the Milvian Bridge, a victory which secured Constantine's position as Augustus of the West. Many who witnessed it would still have been alive when Eusebius wrote his biography. For more information, see: http://www.evolpub.com/CRE/CREseries.html#CRE8