In hoc signo vinces

In hoc signo vinces (Classical Latin: [ɪn hoːk ˈsɪŋnoː ˈwɪŋkeːs]; Ecclesiastical Latin: [in ɔk ˈsiɲɲo ˈvintʃes]) is a Latin phrase meaning "In this sign you will conquer", often also rendered in early modern English as "In this sign thou shalt conquer". It is a translation, or rendering, of the Greek phrase "ἐν τούτῳ νίκα" en toútōi níka (Ancient Greek: [en tóːtɔ͜ːi níːkaː], Greek pronunciation: [en ˈtuto ˈnika]), literally meaning "in this, conquer".

Lucius Caecilius Firmianus Lactantius was an early Christian author (c. 240 – c. 320) who became an advisor to the first Christian Roman emperor, Constantine I, guiding his religious policy as it developed, and tutor to his son. His work De Mortibus Persecutorum has an apologetic character, but has been treated as a work of history by Christian writers. Here Lactantius preserves the story of Constantine's vision of the Chi Rho before his conversion to Christianity. The full text is found in only one manuscript, which bears the title, Lucii Caecilii liber ad Donatum Confessorem de Mortibus Persecutorum.

The historian bishop Eusebius of Caesaria states that Constantine was marching with his army (Eusebius does not specify the actual location of the event, but it is clearly not in the camp at Rome), when he looked up to the sun and saw a cross of light above it, and with it the Greek words "() " ("In this, conquer"), a phrase often rendered into Latin as in hoc signo ("in this sign, you will conquer").

At first, Constantine did not know the meaning of the apparition, but on the following night, he had a dream in which Christ explained to him that he should use the sign of the cross against his enemies. Eusebius then continues to describe the Labarum, the military standard used by Constantine in his later wars against Licinius, showing the Chi-Rho sign. The accounts by Lactantius and Eusebius, though not entirely consistent, have been connected to the Battle of the Milvian Bridge (312 AD), having merged into a popular notion of Constantine seeing the Chi-Rho sign on the evening before the battle.

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