Lactantius

“ The armies met, and fought with the utmost exertions of valour, and firmly maintained their ground. In the meantime a sedition arose at Rome, and Maxentius was reviled as one who had abandoned all concern for the safety of the commonweal; and suddenly, while he exhibited the Circensian games on the anniversary of his reign, the people cried with one voice, “Constantine cannot be overcome!” Dismayed at this, Maxentius burst from the assembly, and having called some senators together, ordered the Sibylline books to be searched. ”

“ He did as he had been commanded, and he marked on their shields the letter Χ, with a perpendicular line drawn through it and turned round thus at the top, being the cipher of Christ. Having this sign (ΧР ) , his troops stood to arms. The enemies advanced, but without their emperor, and they crossed the bridge. ”