In Hebrew, Chi-Rho equates to Tav-Resh. The Chi-rho is also the origin of the tradition of abbreviating "Christ" in "Christian" or "Christmas" to "X."
The etymology of the word before Constantine's usage of it is unclear. According to Lactantius (On the Deaths of the Persecutors, chapter 44), Constantine had dreamed of this emblem and a voice saying "In this sign you shall conquer" (In hoc signo vinces). 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.
Mythology characteristically differs in the details, but in every case, the details are meaningful, never random. Writing in Greek, Eusebius of Caesarea (died 339), the bishop who wrote the first surviving general history of the early Christian churches, gave two additional versions of Constantine's famous vision:
Swedish geologist Jens Ormo suggests that the latter account may have had its origins in Constantine witnessing the mushroom cloud from the meteorite impact that resulted in the Sirente crater situated in Sirente-Velino Regional Park, Abruzzo, Italy *.
In medieval art, the labarum theme is conspicuous by its absence, appearing suddenly in the Renaissance and classical periods, where the phrase is frequently shown written out in the sky.
Eusebius may have felt that the dream mytheme on its own needed reinforcement. Of the 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."
Among the many soldiers depicted on the Arch of Constantine, which was erected just three years after the battle, the labarum does not appear, nor is there any hint of the miraculous affirmation of divine protection that had been witnessed, Eusebius avers, by so many. A grand opportunity for just the kind of political propaganda that the Arch otherwise was expressly built to present, would have been unaccountably missed, if Eusebius' oath-confirmed account can be trusted. Its inscription does say that the emperor had saved the res publica INSTINCTU DIVINITATIS MENTIS MAGNITUDINE ("by greatness of mind and by instinct impulse of divinity"). Which divinity is not identified, though Sol Invictus— the Invincible Sun (also identifiable in Apollo or Mithras)— is inscribed on Constantine's coinage at this moment.
In his Historia Ecclesiae Eusebius further reports that, after his victorious entry into Rome, Constantine had a statue of himself erected, "holding the sign of the Savior cross in his right hand." There are no other reports to confirm such a conspicuous monument.
It 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". Protestant Christians, especially Restorationists, reject its use due to what they believe to be pagan origins—specifically, as a symbol of the sun god—and lack of use by the earliest Christians, 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, however, reinforced by the fact that Julian the Apostate removed it from his insignia, and that it was restored to use by his Christian successors.
The labarum is official symbol of the Alpha Chi Rho National Fraternity.
In Unicode, the Chi-Rho symbol is U+2627 (☧).
There is a conspiracy theory claim that Microsoft Windows XP is named after the Chi-Rho symbol, with 'Cairo' (which is a homonym for Chi-Rho) as the predecessor.*
Alchemy | Ancient Roman Christianity | Christian symbols
Lábaro | Labarum | Labarum | 라바룸 | Labarum | ラバルム | Labarum | Лабарум | labarum | Labarum