In hoc signo vinces

[2] 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"),[3] a phrase often rendered into Latin as in hoc signo vinces ("in this sign, you will conquer").

Eusebius then continues to describe the Labarum,[5] the military standard used by Constantine in his later wars against Licinius, showing the Chi-Rho sign.

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.

[6] 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.

Detail from a 9th-century Byzantine manuscript. Constantine defeats Maxentius at the Milvian Bridge; the vision of Constantine is a Greek cross with ἐν τούτῳ νίκα written on it.
Ceiling of the Church of Saint Anthony of the Portuguese , in Rome , depicting the Miracle at the Battle of Ourique .