Лабарум: Военный штандарт с символом Христа.

Roman military standard displaying XP for Christ
the vexillum

The labarum (λάβαρον or λάβουρον) was a vexillum (military standard) that displayed the "Chi Rho" symbol ☧, a christogram formed from the first two Greek letters of the word "Christ" (ΧΡΙΣΤΟΣ, or Χριστός) – Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine the Great. Ancient sources draw an unambiguous distinction between the two terms "labarum" and "Chi Rho", even though later usage sometimes regards the two as synonyms. The name labarum was applied both to the original standard used by Constantine the Great and to the many standards produced in imitation of it in the Late Antique world, and subsequently.

"A Description of the Standard of the Cross, which the Romans now call the Labarum." "Now it was made in the following manner. A long spear, overlaid with gold, formed the figure of the cross by means of a transverse bar laid over it. On the top of the whole was fixed a wreath of gold and precious stones; and within this, the symbol of the Saviour’s name, two letters indicating the name of Christ by means of its initial characters, the letter P being intersected by X in its centre: and these letters the emperor was in the habit of wearing on his helmet at a later period. From the cross bar of the spear was suspended a cloth, a royal piece, covered with a profuse embroidery of most brilliant precious stones; and which, being also richly interlaced with gold, presented an indescribable degree of beauty to the beholder. This banner was of a square form, and the upright staff, whose lower section was of great length, of the pious emperor and his children on its upper part, beneath the trophy of the cross, and immediately above the embroidered banner." "The emperor constantly made use of this sign of salvation as a safeguard against every adverse and hostile power, and commanded that others similar to it should be carried at the head of all his armies."

The labarum does not appear on any of several standards depicted on the Arch of Constantine, which was erected just three years after the battle. If Eusebius' oath confirmed account of Constantine's vision and the role it played in his victory and conversion can be trusted, then a grand opportunity for the kind of political propaganda that the Arch was built to present was missed. Many historians have argued that in the early years after the battle, the Emperor had not yet decided to give clear public support to Christianity, whether from a lack of personal faith or because of fear of religious friction. The arch's inscription does say that the Emperor had saved the res publica INSTINCTV DIVINITATIS MENTIS MAGNITVDINE ("by greatness of mind and by instinct [or impulse] of divinity"). Continuing the iconography of his predecessors, Constantine's coinage at the time was inscribed with solar symbolism, interpreted as representing Sol Invictus (the Unconquered Sun), Helios, Apollo, or Mithras, but in 325 and thereafter the coinage ceases to be explicitly pagan, and Sol Invictus disappears. And although Eusebius' Historia Ecclesiae further reports that Constantine had a statue of himself "holding the sign of the Savior [the cross] in his right hand" erected after his victorious entry into Rome, there are no other reports to confirm such a monument. Historians still dispute whether Constantine was the first Christian Emperor to support a peaceful transition to Christianity during his rule, or an undecided pagan believer until middle age, and also how strongly influenced he was in his political religious decisions by his Christian mother St. Helena. As for the labarum itself, there is little evidence for its use before 317. In the course of Constantine's second war against Licinius in 324, the latter developed a superstitious dread of Constantine's standard. During the attack of Constantine's troops at the Battle of Adrianople the guard of the labarum standard were directed to move it to any part of the field where his soldiers seemed to be faltering. The appearance of this talismanic object appeared to embolden Constantine's troops and dismay those of Licinius. At the final battle of the war, the Battle of Chrysopolis, Licinius, though prominently displaying the images of Rome's pagan pantheon on his own battle line, forbade his troops from actively attacking the labarum, or even looking at it directly. Constantine felt that both Licinius and Arius were agents of Satan, and associated them with the serpent described in the Book of Revelation (12:9). Constantine represented Licinius as a snake on his coins. Eusebius stated that in addition to the singular labarum of Constantine, other similar standards (labara) were issued to the Roman army. This is confirmed by the two labara depicted being held by a soldier on a coin of Vetranio (illustrated) dating from 350.

A later Byzantine manuscript indicates that a jewelled labarum standard believed to have been that of Constantine was preserved for centuries, as an object of great veneration, in the imperial treasury at Constantinople. The labarum, with minor variations in its form, was widely used by the Christian Roman emperors who followed Constantine. A miniature version of the labarum became part of the imperial regalia of Byzantine rulers, who were often depicted carrying it in their right hands. The term "labarum" can be generally applied to any ecclesiastical banner, such as those carried in religious processions. "The Holy Lavaro" were a set of early national Greek flags, blessed by the Greek Orthodox Church. Under these banners the Greeks united throughout the Greek Revolution (1821), a war of liberation waged against the Ottoman Empire. Labarum also gives its name (Labaro) to a suburb of Rome adjacent to Prima Porta, one of the sites where the 'Vision of Constantine' is placed by tradition.