In hoc signo vinces
"In hoc signo vinces" (Classical Latin: [ɪn hoːk ˈsɪŋnoː ˈwɪŋkeːs], Ecclesiastical Latin: [in ok ˈsiɲo ˈvintʃes]) is a Latin phrase conventionally translated into English as "In this sign thou shalt conquer".
The Latin phrase itself renders, rather loosely, the Greek phrase "ἐν τούτῳ νίκα", transliterated as "en toútōi níka" (Ancient Greek: [en túːtɔːi̯ níːkaː], Modern Greek: [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 (and tutor to his son), guiding the Emperor's religious policy as it developed during his reign. 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 bishop Eusebius of Caesaria, a historian, 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 "(ἐν) τούτῳ νίκα" ("In this, conquer"), a phrase often rendered into Latin as in hoc signo vinces ("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.
The phrase appears prominently placed as a motto on a ribbon unfurled with a passion cross to its left, beneath a window over the Scala Regia, adjacent to the equestrian statue of Emperor Constantine, in the Vatican. Emperors and other monarchs, having paid respects to the Pope, descended the Scala Regia, and would observe the light shining down through the window, with the motto, reminiscent of Constantine's vision, and be reminded to follow the Cross.
- Inscribed on the banner of the Sanfedismo in 1799
- Inscribed in Greek on the flag (obverse side) of the Sacred Band of the Greek War of Independence
- Part of the trademark for Pall Mall cigarettes. "PALL MALL IN HOC SIGNO VINCES 'WHEREVER PARTICULAR PEOPLE CONGREGATE'"  
- It is the public motto of the English Defence League, emblazoned around the group's logo.
- The phrase is the motto on some Byzantine coins (e.g. the folles of Constans II).
- It is the motto on most regimental flags of the Irish Brigade (France).
- In hoc signo vinces is the motto on the O'Donnell coat of arms.
- It is the motto for the College of the Holy Cross
- "Lucius Caecilius Firmianus Lactantius". Online Catholic Encyclopedia. New Advent.
- Roberts, Alexander; Donaldson, James, eds. (1871). "The manner in which persecutors died. Chapter 44". The works of Lactantius. Volume II. Ante-Nicene Christian Library: Translations of the writings of the Fathers. Down to A.D. 325. Vol. XXII. Edinburgh. p. 203.
- Eusebius. "1.28". Vita Constantini (PDF). www.documentacatholicaomnia.eu. p. 944.
- Haaren, John H.; Poland, A. B. (2006) . Famous Men of Rome. Yesterday's Classics. p. 229. ISBN 978-159915-046-8.
- Eusebius, Vita Constantini 1.31, p. 946.
- Howard Hibbard (30 August 1990). Bernini. Penguin Books Limited. p. 122. ISBN 978-0-14-193542-3.
- Chadwick, Owen. 1981. The Popes and European Revolution. Oxford University Press. ISBN 0-19-826919-6. p. 474.
- Helene P. Kokkone; Katerina Korre-Zographou; Chrysa Daskalopoulou (1997). Ελληνικές Σημαίες, Σήματα, Εμβλήματα (in Greek). Athens: G. Tsiberiotes. ISBN 960-7795-01-6.
- "PALL MALL IN HOC SIGNO VINCES WHEREVER PARTICULAR PEOPLE CONGREGATE - Trademark Details". Justia.
- U.S. Customs and Border Protection, Department of Homeland Security. COPYRIGHT, TRADEMARK,AND TRADE NAME RECORDATIONS (No. 12 2015) (PDF) (Report). p. 3.
- Elliott, Andrew B. R. (2017). Medievalism, Politics and Mass Media: Appropriating the Middle Ages in the Twenty-First Century. Medievalism Volume 10. Woodbridge: Boydell. p. 166. ISBN 978-1-84384-463-1.
- Grierson, Philip (1982). Byzantine Coins. University of California Press. p. 111. ISBN 0-520-04897-0.
- Kuzniewsk, Anthony. "Top 25 Moments in Holy Cross History". Retrieved 26 April 2022.
- Eusebius. "1.28". Βίος Κωνσταντίνου [Vita Constantini] (in Greek). At the Internet Archive.
- Eusebius. "1.28". Eusebius - Constantine and the sign of the cross. www.earlychurchtexts.com (in Greek). Passages 1.26-31 of Vita Constantini.
- Eusebius. "1.28". Vita Constantini (PDF). www.documentacatholicaomnia.eu (in Latin). Vol. Book 1. p. 7 (21–22 on scanned book).
- Lactantius. "Lucii Caecilii liber ad Donatum Confessorem de Mortibus Persecutorum". www.thelatinlibrary.com (in Latin).
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