The Labarum (☧) was a military standard that displayed the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός) — Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine I.
The etymology of the word before Constantine's usage of it is unclear.
According to Lactantius, a historian of North African origin saved from poverty under the patronage of Constantine Ι as tutor to his son Crispus, who was writing in Latin, Constantine had dreamt of this emblem and a voice saying In hoc signo vinces ("In this sign you shall conquer"). On waking he ordered his soldiers to put the emblem on their shields; that very day they fought the forces of Maxentius and won the Battle of the Milvian Bridge (312), outside Rome.
Writing in Greek, Eusebius of Caesarea (died in 339), the bishop who wrote the first surviving general history of the early Christian churches, gave the two definitive versions of Constantine's famous vision, accepted by the Orthodox Churches, by which Constantine I was later canonized for his contributions to Christianity as a saint:
Eusebius may have felt that the dream mytheme on its own needed reinforcement supporting his emperor's account, presenting it as a true miracle. He wrote in the Vita that Constantine himself had told him this story "and confirmed it with oaths," late in life "when I was deemed worthy of his acquaintance and company." "Indeed," says Eusebius, "had anyone else told this story, it would not have been easy to accept it."
Constantine's modern biographer, Ramsey MacMullen, is one who finds Eusebius' justification not easy to accept: "If the sky writing was witnessed by 40,000 men, the true miracle lies in their unbroken silence about it".
There are numerous modern astronomical and astrological theories that defend Eusebius' account as possible. In 1948 Fritz Heiland, of the Zeiss planetarium at Jena, published his astronomical interpretation of Constantine's vision, that the fall of the year 312 was attended by an unusual spectacle: the syzygy or close alignment of three bright planets, Mars, Saturn and Jupiter, in the evening sky above the southwest horizon, positioned along a line within about 20 degrees of each other on the border of Capricorn and Sagittarius. Heiland suggests that Constantine overcame the psychological impact on his army, of the ill pagan content of the astrological omen that associated syzygies with bad outcomes, by appropriating it to fashion a Christian token of victory in the form of the labarum.
The Swedish geologist Jens Ormo and co-authors suggest that the account may have had its origins in Constantine's witnessing the daylight effects of a meteorite's descent through earth's atmosphere, of which the impact he believes resulted in the Sirente crater situated in Sirente-Velino Regional Park, Abruzzo, Italy
Whether Constantine I was the first Christian emperor supporting during his rule a peaceful transition to Christianity or an undecided pagan believer until middle age, strongly influenced in his political-religious decisions by his Christian mother St. Helena is still in dispute among historians. He is celebrated together with his mother St. Helena as Equal-to-apostles (isapostoloi) on 21 May by both the Eastern Orthodox Church and Oriental Orthodoxy as a saint of Christianity. but the Latin Roman Catholic Church, while celebrating Helena of Constantinople as a saint, has never placed Constantine among the saints, but has been content with naming him "the Great," in grateful remembrance of his services to the cause of Christian civilization.
He definitely bestowed imperial favor on Christianity by returning and legally protecting thereafter property taken from the early church. He offered legal protection of the freedom of confession to all religions within the Empire, with the famous Edict of Milan in 313 and not only "toleration of other religions as indulgence" as Galerius did in 311. Constantine I was the first to declare (7 March 321) dies Solis (Day of the Sun, Sunday) as the day of rest throughout the Empire. He declared Rome a Christian city and allegedly placed his mother in charge of locating Christian relics, resulting in the discovery of the True Cross in the Holy Land. A small portion of the relics that she located (together with the Nails), two of which she later gave to her son Constantine for protection, together with soil from the True Cross excavation site and big parts of the Cross itself were then stored in a room inside her palace in Rome around which the basilica of Santa Croce in Gerusalemme - housing the Passion relics - was built (also named St. Helena's chapel). It was later (15th century) converted into the Abbey of Santa Croce. The famous relics, whose authenticity is disputed, are now housed in a chapel (the Cappella delle Reliquie), built in 1930 by architect Florestano di Fausto. They also include: a part of the Elogium or Titulus Crucis, i.e. the panel which was hanged to the Christ's Cross; two thorns of his crown; the third nail (incomplete); and three small wooden pieces of the True Cross itself. A much larger piece of the holy cross was brought from Santa Croce in Gerusalemme to St. Peter's Basilica on instruction of Pope Urban VIII in the year 1629. It is kept near the statue of St. Helena, made by Andrea Bolgi in 1639.
As for the labarum, its first appearances to which a precise date can be assigned are numismatic: the usurper Magnentius appears to have been the first to use the chi-rho monogram flanked by Alpha and Omega, on the reverse of some coins minted in 353. In Roman Britannia, a tesselated mosaic pavement was uncovered at Hinton St Mary, Dorset, in 1963:. On stylistic grounds it is dated to the fourth century; its central roundel represents a beardless male head and bust draped in a pallium in front of the chi-rho symbol, flanked by pomegranates, symbols of eternal life.
The labarum has since been interpreted by Christians all over the world as a symbol of Christianity. Because it is composed of the combined chi and rho it is sometimes referred to as the "monogram of Christ". Some Protestant Christians, especially Restorationists, reject its use due to what they believe to be pagan origins, although it was already in widespread use by Christians in the 3rd century, mostly on sarcophagi.
The interpretation of its use as a specifically Christian symbol is reinforced by the fact that Julian the Apostate removed it from his insignia, and that it was restored to use by his Christian successors.
"Labarum" is also used for any ecclesiastical banner, such as those carried around in processions as well as under the name "the holy lavaro" for the set of early national Greek flags, blessed by the Greek Orthodox Church, under which the Greeks united, from the commencement and throughout the Greek Revolution (1821) against the Ottoman Empire, which was occupying Greece at the time.