From the time of the Roman Emperor, Constantine the Great (306-337), it was the imperial and military banner (a vexillum). The original standard was first used by Constantine during the Battle of the Milvian Bridge against Maxentius (312) but the same name also refers to similar standards produced in imitation of the original one in the Late Antique world and later on.
The origin of the word labarum is a matter of scientific debate. Some suggest it derived from the Latin word labarum. Others say it is a Gallic word because Gaul was the starting point for the war against Maxentius, and there were many Gauls in the army.
After a Christian author and an advisor to Roman emperor, Constantine I, Lactantius (240-320), shortly before the battle, the emperor fell into ecstasy, during which he received an order from Christ to place on the shields of soldiers the sign of heaven, consisting of the first two Greek letters of the word ‘Christ’ (ΧΡΙΣΤΟΣ) — Chi (χ) and Rho (ρ). This monogram is indeed found on coins and writings from the time of Constantine. Eusebius of Caesarea (260-339), a biographer of the emperor’s life, adds that at the moment of the start of the fight, the pagan ruler called for the help of the Christian God, as a result of which he saw in the daylight a radiant cross with the Greek words: ‘Through this sign you will win!’ The next night, Constantine saw Christ with the cross and was advised to have a banner made with the image of the cross, displaying the christogram of the ‘Chi-Rho’ symbol ☧. This banner is, of course, a labarum, made in the shape of the letter T, standing for the cross, and attached to the upper bar. As such it was henceforth carried by Constantine’s troops.
The day after his victory, on October 29, 312, Constantine rode triumphantly into Rome. The city gave him a wonderful party. As for the vaccination of Christianity, it was still until the end of his reign, a transitional period. But although Constantine chose not to tease the Roman pagans with a new religion in one God, he nonetheless openly manifested the origins of Christianism in the heart of the Western world by minting coins with the christogram of the ‘Chi-Rho’ while the labarum in the form of the cross flew over the ranks of his army.
Featured image: The emperor Constantine IX Monomachos (1042-1055) (centre panel of a Byzantine enamelled crown) holding a miniature labarum. Photo by Andrew massyn (2008). Public domain. Image cropped. Colours intensified. Photo source: “Labarum” (2020). In: Wikipedia. The Free Encyclopedia.
“Labarum” (2020). In: Wikipedia. The Free Encyclopedia. Available at <https://bit.ly/3tPYsM8>. [Accessed 12th February, 2021].
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Rops D. (1968). Kościół pierwszych wieków. pp. 477-480. [L’église des apôtres et des martyrs]. Ostrowska K. trans. Warszawa: Instytut Wydawniczy PAX.