Constantine the Great's Vision of the Cross

“While he was thus praying with fervent entreaty, a most marvelous sign appeared to him from heaven, the account of which it might have been hard to believe had it been related by any other person. But since the victorious emperor himself long afterwards declared it to the writer of this history, when he was honored with his acquaintance and society, and confirmed his statement by an oath, who could hesitate to accredit the relation, especially since the testimony of after-time has established its truth? He said that about noon, when the day was already beginning to decline, he saw with his own eyes the sign of a cross of light in the heavens, above the sun, and bearing the inscription, 'By this symbol you will conquer.' He was struck with amazement by the sight, and his whole army, which accompanied him on this expedition, and witnessed the miracle, was astonished. 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; 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. At the dawn of day he arose, and communicated the secret to his friends: and then, calling together the workers in gold and precious stones, he sat in the midst of them, and described the figure of the sign and bade them represent it in gold and precious stones. And this representation the emperor himself was accustomed to wear on his helmet at a later period.”

— Eusebius of Caesarea, Life of Constantine, Book I, Chapters 28–29