The Chi Rho is one of the earliest forms of christogram, and is used by some Christians. It is formed by superimposing the first two (capital) letters chi and rho (ΧΡ) of the Greek word "ΧΡΙΣΤΟΣ" =Christ in such a way to produce the monogram. Although not technically a Christian cross, the Chi-Rho invokes the crucifixion of Jesus, as well as symbolizing his status as the Christ.
The Chi-Rho symbol was also used by pagan Greek scribes to mark, in the margin, a particularly valuable or relevant passage; the combined letters Chi and Rho standing for chrēston, meaning "good." Some coins of Ptolemy III Euergetes (r. 246–222 BC) were marked with a Chi-Rho.
Christian accounts of Constantine's adoption of the Chi-Rho 
According to Lactantius, a Latin historian of North African origins saved from poverty by the patronage of Emperor Constantine I (r. 306–337) as tutor to his son Crispus, Constantine had dreamt of being ordered to put a "heavenly divine symbol" (Latin: coeleste signum dei) on the shields of his soldiers. The description of the actual symbol chosen by Emperor Constantine the next morning, as reported by Lactantius, is not very clear: it closely resembles a Chi-Rho or a staurogram, a similar Christian symbol. That very day Constantine's army 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 two different accounts of the events. In his church history, written shortly after the battle, when Eusebius didn't yet have any contact with Constantine, he doesn't mention any dream or vision, but compares the defeat of Maxentius (drowned in the Tiber) to that of the biblical pharaoh and credits Constantine's victory to divine protection.
In a memoir of the Roman emperor that Eusebius wrote after Constantine's death (On the Life of Constantine, circa 337–339), a miraculous appearance came in Gaul long before the Battle of the Milvian Bridge. In this later version, the Roman emperor had been pondering the misfortunes that befall commanders that invoke the help of many different gods, and decided to seek divine aid in the forthcoming battle from the One God. At noon, Constantine saw a cross of light imposed over the sun. Attached to it, in Greek characters, was the saying "Τούτῳ Νίκα!". Not only Constantine, but the whole army saw the miracle. That night, Christ appeared to the Roman emperor in a dream and told him to make a replica of the sign he had seen in the sky, which would be a sure defence in battle.
Eusebius 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."
Celestial chi 
Although modern representations of the Chi-Rho sign represent the two lines crossing at ninety degree angles, the early examples of the Chi-Rho cross at an angle that is more vividly representative of the chi formed by the solar ecliptic path and the celestial equator. This image is most familiar in Plato's Timaeus, where it is explained that the two bands which form the "world soul" (anima mundi) cross each other like the letter chi. Not only did the two legs of the chi remind early Christians of the Holy Cross, "it reminded them of the mystery of the pre-existent Christ, the Logos Theou, the Word of God, who extended himself through all things in order to establish peace and harmony in the universe," in Robert Grigg's words. Hugo Rahner summarized the significance: "The two great circles of the heavens, the equator and the ecliptic, which, by intersecting each other form a sort of recumbent chi and about which the whole dome of the starry heavens swings in a wondrous rhythm, became for the Christian eye a heavenly cross." Of Plato's image in Timaeus, Justin Martyr, the Christian apologist writing in the 2nd century, found a prefiguration of the Holy Cross, and an early testimony may be the phrase in Didache, "sign of extension in heaven" (sēmeion ekpetaseōsen ouranō).
An alternate explanation of the intersecting celestial symbol has been advanced by George Latura, claiming that Plato's visible god in Timaeus is in fact the intersection of the Milky Way and the Zodiacal Light, a rare apparition important to pagan beliefs that Christian bishops reinvented as a Christian symbol.
Later usage 
The use of a wreath around the Chi-Rho symbolizes the victory of the Resurrection over death, and is an early visual representation of the connection between the Crucifixion of Jesus and his triumphal resurrection, as seen in the 4th century sarcophagus of Domitilla in Rome. Here, in the wreathed Chi-Rho the death and resurrection of Christ are shown as inseparable, and the Resurrection is not merely a happy ending tucked at the end of the life of Christ on Earth. Given the use of similar symbols on the Roman standard, this depiction also conveyed another victory, namely that of the Christian faith: the Roman soldiers who had once arrested Jesus and marched him to Calvary now walked under the banner of a resurrected Christ.
After Constantine, the Chi-Rho became part of the official imperial insignia. Archaeologists have uncovered evidence demonstrating that the Chi-Rho was emblazoned on the helmets of some Late Roman soldiers. Coins and medallions minted during Emperor Constantine's reign also bore the Chi-Rho. By the year 350, the Chi-Rho began to be used on Christian sarcophagi and frescoes. 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 4th 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. Another Romano-British Chi-Rho, in fresco, was found at the site of a villa at Lullingstone (illustrated). The symbol was also found on Late Roman Christian signet rings in Britain.
See also 
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- Steffler 2002, p. 66.
- Southern 2001, p. 281; Grant 1998, p. 142.
- von Reden 2007, p. 69: "The chi-rho series of Euergetes' reign had been the most extensive series of bronze coins ever minted, comprising eight denominations from 1 chalkous to 4 obols."
- Lactantius. On the Deaths of the Persecutors, Chapter 44.
- The well known sentence In hoc signo vinces is simply a later Latin translation of Eusebius's Greek wording.
- Eusebius Pamphilius: Church History, Life of Constantine, Oration in Praise of Constantine, Chapter 31.
- Plato. Timaeus, 8.36b and 8.36c: "And thus the whole mixture out of which he cut these portions was all exhausted by him. This entire compound divided lengthways into two parts, which he joined to one another at the centre like the letter X, and bent them into a circular form, connecting them with themselves and each other at the point opposite to their original meeting-point; and, comprehending them in a uniform revolution upon the same axis, he made the one the outer and the other the inner circle."
- Grigg 1977, p. 477.
- Rahner & Battershaw 1971, "Mystery of the Cross", pp. 49–50. See also Grigg 1977, p. 477 (including Note #59)
- Justin. Apologia, 1.60.
- Noted by Grigg 1977, p. 477 (Note #42).
- Latura, G. "Plato’s Visible God: The Cosmic Soul Reflected in the Heavens." Religions 2012, 3, 880-886. http://www.mdpi.com/2077-1444/3/3/880
- Harries 2004, p. 8.
- Kellner 1968, p. 57ff. See also Grigg 1977, p. 469 (Note #4).
- Johns 1996, p. 67.
- Grant, Michael (1998). The Emperor Constantine. London, United Kingdom: Phoenix Giant. ISBN 0-7538-0528-6.
- Grigg, Robert (December 1977). ""Symphōnian Aeidō tēs Basileias": An Image of Imperial Harmony on the Base of the Column of Arcadius". The Art Bulletin 59 (4): 469–482.
- Harries, Richard (2004). The Passion in Art. Burlington, Vermont: Ashgate Publishing Company. ISBN 0-7546-5011-1.
- Heiland, Fritz (1948). "Die astronomische Deutung der Vision Kaiser Konstantins". Sondervortrag im Zeiss-Planatarium-Jena: 11–19.
- Johns, Catherine (1996). The Jewellery of Roman Britain: Celtic and classical Traditions. Oxon, United Kingdom: Routledge (Taylor & Francis Group). ISBN 1-85728-566-2.
- Kellner, Wendelin (1968). Libertas und Christogramm: Motivgeschichtliche Untersuchungen zur Münzprägung des Kaisers Magnentius (350-353) (in German). Karlsruhe, Germany: Verlag G. Braun.
- Rahner, Hugo; Battershaw, Brian (translator) (1971). Greek Myths and Christian Mystery. New York, New York: Biblo & Tannen Booksellers and Publishers Incorporated. ISBN 0-8196-0270-1.
- Southern, Pat (2001). The Roman Empire from Severus to Constantine. New York, New York and London, United Kingdom: Routledge (Taylor & Francis Group). ISBN 0-415-23943-5.
- Steffler, Alva William (2002). Symbols of the Christian Faith. Grand Rapids, Michigan and Cambridge, United Kingdom: Wm. B. Eerdmans Publishing. ISBN 0-8028-4676-9.
- von Reden, Sitta (2007). Money in Ptolemaic Egypt: From the Macedonian Conquest to the End of the Third Century BC. Cambridge, United Kingdom: Cambridge University Press. ISBN 0-521-85264-1.