The origin of the Chi Rho sign before Costantine's usage is unclear (Nevertheless, see H. Grégoire, "L'étymologie de 'Labarum'" Byzantion 4 (1929:477-82).

According to Lactantius [Lactantius, On the Deaths of the Persecutors, chapter 44.] a historian of North African origin saved from poverty and under the patronage of Constantine Ι as tutor to his son Crispus, who was writing in Latin, roman emperor Costantine Ist made a new military emblem, inspiration by a vision he dreamed of this simbol and a voice saying Εν τουτο νικα, alias In hoc signo vinces, ["Labarum (Chi-Rho)". Catholic Encyclopedia. (1913). New York: Robert Appleton Company.] means in this sign you shall conquer [^ Lactantius, On the Deaths of the Persecutors, chapter 44.], so Costantine ordered that Chi Rho emblema had to be draw to his soldiers shields; that very day they fought the forces of Maxentius and won the Battle of the Milvian Bridge (312), outside Rome.

Costantine however used to ascribe to himself title of Pontifex Maximus, that is the higest pagan religion exponent, thus many historian deducted Costantine never really had convert to Christian religion, but he only had political interest to Christianity

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:

However, between the soldier represented on the triumphal Arch of Constantine erected just 3 years after the battle, labarum doesn't appear, and there is no evidence about the miralce God's protection recorded by Usebio, moreover an inscription on the triumphal Arch reports that emperor Constantine saved res publica instinctu divinatis mentis magnitudine, but there's no specification about which divinity is talking about, in exemple conio costantiniano of the same period is wrote - sol invictus - , title attribed both to Apollo and Mitra god.

In Historia Ecclesiae Eusebio reported that, after he entered triuphing in Rome, Constantine ordered to erect a statue representing himself holding the sign of the Saviour, the crux, by his right hand, but sculpture never had to be found and there are no other historical records reporting about this monument.

However Chi Rho simbol used as specify Christian icon seems to be confirmed by Giuliano emeperor measure to remove Chi Rho from his emperial flags and insignia, where Chi Rho was restabilished by Giuliano's christian successors.

Eusebius may have felt that the dream mytheme on its own needed reinforcement supporting his emperor's account, presenting it as a true miracle sow from whole Constantine army, so 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:[MacMullen, Constantine, 1969.] "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 [ Ross J. David, 2000 "The Bird, The Cross, And The Emperor. Investigations into The Antiquity of The Cross in Cygnus" http://www.twcac.org/onlinehorizon/cross.htm -> August 5th, 2008 ] that defend Eusebius' account as possible. In 1948 Fritz Heiland, of the Zeiss planetarium at Jena, published[ Heiland, “Die astronomische Deutung der Vision Kaiser Konstantins”, Sondervortrag im Zeiss-Planatarium-Jena 1948:11-19. ] his astronomical interpretation of Constantine's vision, that the fall of the year 312 was attended by an unusual spectacle: the σύζυγος - syzygos, Latin 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 suggested [Whitehouse D., 2003 "Space impact 'saved Christianity' " http://newsvote.bbc.co.uk/mpapps/pagetools/print/news.bbc.co.uk/2/hi/science/nature/3013146.stm ->August 5th, 2008] 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.

In Wester Europe Christianity, both Catholics and many Protestants, Christ Chi Rho monogram was supplied by acronym IHS or JHS, made by the first three letters og Greek name Jesus - iota-eta-sigma; here Greek letter eta was traslittered as Latin H, probably because of similar ortography even Greek sigma letter was translated in Latin S or C, moreover in medieval Latin there was no differents between J and I letters, so Christ monogram could be both wrote as JHS, JHC, IHS, IHC.

Acronym IHS sometimes is iterpreted as Iesus Hominum Salvator - Jesus men's savior abbreviation, or In Hoc Signus [vinces] regarded to Constantine's vision.

Particular and great propulsion to HIS Christ monograms diffusion could be attribute to great preaching opera of Bernardino from Siena, who diffused this iconography among common people: according to tradition while Bernardino spoke and preached, believeres used to kiss some HIS surmonted by crux and circled by a sun carved wooden tables

Ortodox Christian Church most use ICXC Christ monogram, made by the first and last 2 letters of ΙΗΣΟΥΣ ΧΡΙΣΤΟΣ - Jesus Christ words; notice that the last Greek letter Σ sigma is similar to Latin C ketter.

In Ortodox icons monogram could be devided in IC at the left side of sacred image, and XC on right side

Ather protestant christian confessions, in particular Restorationism, sometimes called Christian primitivism, refuse tu use such monogram because they belived it was not used by ancient Christian and they assign Christ Monogram to pagan origins, as a sun god symbol.