constantine. I. The Life of Constantine. II. The Oration of Constantine. III. The Oration of Eusebius · The Life of the Blessed Emperor Constantine, by Eusebius Pamphilus · Chapter XXIX
How the Christ of God appeared to him in his Sleep, and commanded him to use in his Wars a Standard made in the Form of the Cross
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He said, moreover, that he doubted within himself what the import of this apparition could be. And while he continued to ponder and reason on its meaning, night suddenly came on; then in his sleep the Christ of God appeared to him with the same sign which he had seen in the heavens, and commanded him to make a likeness of that sign which he had seen in the heavens, and to use it as a safeguard in all engagements with his enemies.
Nicene and Post-Nicene Fathers, Series II, Volume 1, ed. Philip Schaff and Henry Wace (1890–1900). Public domain. Text from the Christian Classics Ethereal Library.